But did you use him or are you just throwing your weight around? In my playthrough, at level 58 or so, my Golurk took on approximately... 30-40% of the Elites without fainting much at all. As in, he didn't faint any more than anybody else did. He was level 60 by the end.
The hardest opponent to defeat was definitely Chandelure. My team in general was very slow, and I think I barely defeated it because I forgot to use Revives after feeding it most of my team as death fodder... adrenaline rush, I suppose. I think the problem was, Walrein was taking near 50% from Shadow Ball, and Volcarona had no way to do much damage. I think I swapped Flamethrower for Psychic, perhaps, and then re-taught him Flamethrower afterwards...
I'd also like to note that I played my game on SET all the way until I got to White Treehollow, where I started playing on SWITCH mode. I was definitely not going easy on myself, or my Pokemon.
I'd love to do another playthrough for you guys, but there's no way I'm deleting my save file after I put so much work into it.
EDIT: Sorry, I did disregarded the format in the OP. For your standards:
Samurott - Mid
Availability: yup
Stats: No one stat jumps out at you. It's not fast, it has average defenses, its attack stats are good but not great. If it had more bulk or speed, the somewhat high offensive stats would be better, but since the whole package is so meh... Most other Water-types do something this guy does better.
Typing: Pure Water is a good type. Coverage attacks are all weak, though.
Move Pool: Just enough to get by. Ubiquitous Surf, Ice Beam... Megahorn is nice, but I already had two Pokemon with Bug-type attacks. I suppose if you had no other physical Bug-type users, it would be great.
Major Battles: Does fine against Cheren's gym. Does fine against Roxie's gym. Does kinda bad against Burgh's gym. Does horrible against Elesa's gym. Does well against Clay's gym. Does fine against Skyla's gym. Dropped him for Walrein before either before or after Drayden. Just... such a boring Pokemon to use. All his level-up move are horrible after Razor Shell.
Misc.: Torrent sucks.
Walrein - Mid
Availability: About halfway through. Gotta set your clock to December or some such, however. That requires no effort at all and only disables daily events for less than 24 hours, but perhaps some people are put off by that? Spheal and Sealeo are plentiful and come in a variety of levels.
Stats: One of the bulkiest Water-types available... way bulkier than Samurott. Attack and Special Attack are both mediocre, but not low. Slower than Samurott without a +Speed nature.
Type: The added weaknesses and reduced resistances of Water/Ice + Thick Fat suck, as opposed to pure Water.
Move Pool: Crunch is nice. Surf & Ice Beam, but the STAB on Ice Beam is really fantastic! Body Slam is way better than anything Samurott has. Gets Encore, just like Samurott, and Hail, too, in case you got Ice Body + Blizzard. Rock Slide is always good. Learns a lot of HMs. Spend some shards and pick up Super Fang if it pleases you.
Major Battles: Destroys most of Drayden's gym, can slowly but surely wear down Marlon's gym with Body Slam and Rock Slide. Go all out on Carracosta, though. Does well against Shauntal and Iris, but horribly against Marshal. Does well against Ghetsis.
Misc.: Thick Fat makes it worth it, otherwise I'd say no way Jose. WAY, WAY easier to come by than Lapras, nearly the same stats, and resistance to Fire is way better than immunity to Water in a game with so few Water-types running around.
Scolipede - Low
Availability: Right after Cheren. Not rare, and the doubles grass means you'll run into them more often.
Stats: Great Defense early on is a big help, and he's one of the fastest early Pokemon. Attack is a bit low for an only-physical attacker, so Adamant is almost required. Low HP is a huge let-down considering how FAT Scolipede is.
Type: Bug/Poison has some good resistances. The weaknesses aren't bad, considering he has no double weaks, unlike most bugs.
Move Pool: Starts out fantastic, lets you down right at the end. Protect, Poison Tail, Rollout, and Bug Bite are great early on, but lack of Poison STAB later on is a HUGE let-down. In BW1, the Poison Jab TM was found way earlier... Megahorn, Rock Slide, and later, Earthquake, are all fantastic. Double Edge makes fighting Fighting-types bearable, but the recoil is awful with such a low HP stat. Superpower can be useful on some teams, but still isn't useful for Fighting-types because of the Defense drop... although it lets you maul Steels before you get Earthquake, I guess. Toxic by level is fantastic, though. Aqua Tail is pretty good, but expensive for only 90 BP.
Major Battles: Great against both Roxie and Bugh, so-so against Elesa and Clay, bad against Skyla, fine against Drayden (the high speed is welcome against his slow Pokemon), Marlon, and Ghetsis. Great against Grimsley; can do backup duty on Caitlin, Marshal, and Iris.
Misc.: No Poison STAB lets you down once Poison Tail loses steam.
Ampharos - Mid
Availability: Right at the beginning, and although not common, is not as rare as Riolu.
Stats: Slow, somewhat bulky, good Special Attack. Starts out frail, though.
Type: Pure Electric is fine. Ground attacks are rare in Unova, outside Driftveil Gym.
Move Pool: Thunder Wave is the big selling point, though later on, you'll probably let it go for more coverage. Decent Electric STAB outside of Volt Switch is somewhat annoying for a while, leaving you with Thunder Punch. Discharge, Fire Punch, Power Gem, Signal Beam, Light Screen, Confuse Ray and Cotton Guard are all great attacks, and all by level-up.
Major Battles: Invaluable against Cheren due to Static and Thunder Wave. Destroys Skyla and Marlon. Useless against Clay, bad against Drayden outside paralysis. After Marlon, usefulness is situational... nothing particular stands out.
Misc.: Static is a great ability. Horrible Speed sucks. For whatever reason, doesn't seem to put out much damage throughout the game.
Lucario - ???
Dropped him early on for Volcarona. Surprised at how pitiful he seemed without high BP Fighting STAB or priority.
Volcarona - Mid
Availability: Guaranteed encounter, but you have to walk through a lot of caves to find him, which is annoying with a full party. Bring 5 or less Pokemon. Shows up right after Clay, I believe. Not having to raise Larvesta is a boon.
Stats: Amazing. Kills everything, fast, not very frail at all.
Type: Bug/Fire is great defensively and offensively, with only the double Rock weakness hindering it, really.
Move Pool: Start out with nothing, really. Once you get Fire Blast or Signal Beam, however, it begins. Everything must die... and then you get Quiver Dance, Flamethower, Psychic, Roost, and Giga Drain. Shame Hurricane is level 90.
Major Battles: With Fire Blast, does well as backup against Skyla with its high speed, though of course it will fall by itself. Drayden and Ghetsis are dealt with if you picked up Signal Beam. Marlon will kill it. Destroys Caitlin by itself. If you want, you can also destroy Grimsley and Marshal solo, too. Thanks, Quiver Dance.
Misc.: Flame Body is very nice, since there's no space for Will-O-Wisp. Give him a Rocky Helmet! Surprisingly good item choice for him.
Reuniclus - Hmmm... Mid
Availability: Quite common. As soon as you reach Nimbassa, you can get one. White 2 only, though.
Stats: Starts out with high Special Attack, and keep on rolling in that regard. Defenses are awful at first, which can be a problem at times, but they end up fantastic in the end. Slower than almost everything. The Speed can be frustrating at times.
Type: Pure Psychic is lame. No bones about it. Weaknesses aren't played on much, though.
Move Pool: Fantastic. Psyshock right off the bat is great. Puts Munna to shame in BW1. Shadow Ball, Energy Ball, Thunder, Reflect, Light Screen, RECOVER, Thunder Wave... all you need.
Major Battles: Nice backup for Elesa. It doesn't care about paralysis, since it's so slow to begin with. Hits all the gyms for at least neutral damage. Get Energy Ball in your hometown, and make Marlon cry. Marshal and Caitlin will fight you hard, but will go down. Not great against Iris, but not horrible. Grimsley & Shauntal will have you for breakfast.
Misc.: Magic Guard is amazing. With this and Recover, it is bulkier than any other Psychic-type in Unova, arguably. Probably hits the hardest, too! Lack of resistances means he doesn't support the team synergy much at all, however... which is, honestly, my only real gripe, albeit a valid one to consider above all else, and for this and the weakness of Solosis alone I give him Mid. High tier otherwise.
Metagross - Mid
Availability: You gotta beat Marlon and chase Team Plasma around a bit before you're privy to the deepest parts of the Giant Chasm, which is getting a bit late in the game. Comes at a high level, so it won't be a Metang for long!
Stats: Comes ready for evolution. Beastly Attack and Defense, and the rest is good, too -- even Special Attack.
Type: Steel/Psychic comes with great resistances, of course, but the offensive STAB combo isn't anything to brag about. Neutrality to Dark and Ghost kinda sucks for a Steel-type, considering Ghetsis and the Elite Four.
Move Pool: Meteor Mash, Psychic, Bullet Punch, Rock Slide, Shadow Ball, Hammer Arm, Zen Headbutt. Quite nice, only lacking a move to overwhelm the masses. Meteor Mash just... is good, but not great, for some reason.
Major Battles: Can help against Ghetsis, though he by no means overwhelms him. Caitlin is helpless against him, for the most part. Does well against the entire Elite Four + Champion, though not overly so due to lack of Dark, Fighting & Ghost resistance. Really helps against Kyurem, btw, if he gives you trouble.
Misc.: Clear Body is a good in-game ability. Metagross seems like he ought to be able to tear things apart, but in fact he's just very solid all around. Good team synergy, as opposed to Reuniclus and most other Psychics. If only Metang came earlier...
Flygon - Low
Availability: You can get a Trapinch as soon as you beat Burgh, though the smart ones will delay gratification and pick up a Vibrava outside Reversal Mountain. Unless... you're a glutton or punishment. That's like, the cutoff for adding Pokemon to your team and still getting a high tier, I guess. Reversal Mountain, I mean. Trapinch are quite rare, but Vibrava are common.
Stats: From hideous, to bad, to good. All that toil... is it worth it?
Type: Ground/Dragon with Levitate is fantastic. His sole redeeming quality. Will fit on any team. The much-coveted Rock resist. Levitate is great for the people who can't suffer a Flying-type on their team.
Move Pool: Somewhat good. The problem is that the moves you want all come REALLY late or post-game: Earthquake, Dragon Claw, Outrage. Earth Power and Dragon Breath/Tail are weaksauce. Rock Slide and Crunch are nothing special at this point. If you buy the expensive Roost for Flygon, it helps keep him relevant, but... Can learn Fly, but is too weak to use it well. Learns a bunch of other stuff, but mostly Special attacks...
Major Battles: Even against Elesa, the horrible defenses of Trapinch will let you down. Same for Clay and Skyla. By the time you get to Drayden, he has a Flygon that's probably at a higher level, faster, and stronger than yours. Flygon really does badly against Drayden, though he can help clean up with a +Speed nature, like mine had. Doesn't help much against Marlon. Doesn't help much at all against anybody. Too weak for too long.
Misc.: If you got Vibrava earlier than level 35, and Flygon earlier than 45, and if Game Freak hadn't taken away Dragon Claw at level 45... but alas, none of those things are true. He looks cool, though.
Golurk - Mid
Availability: Commonly found in the first room 1 of Victory Road. If you're unhappy with a member of your team, or if you've been travelling with an HM slave and want backup for the Elite Four, hey, now's your chance: Golurk is the last of the great Pokemon opened up to you until after beating the Champion.
Stats: Great. Higher than average Attack, good defenses, but slow. 55 base Speed isn't dead slow, however.
Type: Ground/Ghost resists Rock, Fighting, Electric, Poison, Bug, and Normal. Common weaknesses to Water, Grass, Ghost, Dark, and Ice can be a problem if your team can't support them, but if you have no answer to any of those type by now... you are in big trouble, bud.
Move Pool: One Heart Scale buys you Shadow Punch, which is all you need to give Shauntal nightmares. If you have some red shards, give him Ice Punch and give Iris a run for her money. Earthquake is invaluable, and other than Excadrill, Sandslash or Camerupt, he may be the only Pokemon on your team to have it by this point. Fly, coupled with that huge Attack, will defeat Marshal by itself. Rock Slide, Drain Punch, Superpower, Fire Punch or Thunder Punch might be of use, but I think the first four moves I mentioned are the best. If you have no more use for Fly, he gets plenty of stuff to replace it with.
Major Battles: Bye, Marshal. Most of Caitlin's team will fall to one Shadow Punch to the face. Shauntal depends on your level: her own Golurk and Drifblim will likely be easy, but you might not be able to quite overtake Cofagrigus or Chandelure. Grimsley is not a total wash, either: depending on your health level and stats, you may be able to defeat Bisharp and Scrafty. Even though Liepard is weak, it's guaranteed to hit you first, and that's going to dent your HP too much -- you'll be gimped right there. Iris' Hydreigon will maul you, so avoid it. Druddigon, Archeops, and Aggron are all fair game to Golurk. Haxorus is a pretty even match. You probably won't finish it off, but one Ice Punch to the face will leave it badly limping. Hit it until Iris stops healing it or it kills you, then finish it off with another Pokemon, most likely.
Misc.: You have to get Iron Fist. Golurk is like a last-minute cram session before an exam that saves your butt, I suppose. A good choice for the "last" battles of the game, no matter what your other teammates may look like. Plus, I mean... who doesn't love 9'2" tall ghost robots made of stone?