Hardest Relative NPC

Oh I remember Leaf Green in Victory Road there was a Blissey? And this was when I didn't know about Special Attack and Special Defense and just stats in general, so i had no idea what a fat blubbery shit Blissey is. And so I could not kill it and eventually I blacked out all because I couldn't kill a Blissey fml. Most frustrating thing ever.

when i was a noob with only a charizard and some level 20 pidgeys i had so much trouble with the elite 4. especially the water/ice one. doesn't mean its hard though, just if your clueless you will have a lot of trouble

ghetsis hydreigon in black was a bitch to take down. it has so many great coverage moves like surf and focus blast which destroy your team. i had an emboar but it was outsped and kod by surf, and the only fighting move it knew was double kick. ghetsis in black 2 was crap though, he had all physical moves

Ghetsis in Black and White. First playthrough, I literally had nothing that could outspeed his Hydriegon, (my team was fairly under leveled, my own fault for training a dieno, and getting a Larvesta from a friend), so the blasted thing could sweep my entire team. Finally I realized I had Dragon Dance on Haxorus, and set up in Cofagrigus's face. Admitantly, Hydriegon is less frightening when you have a +6 Haxorus on the field.
Red on any game. G/S/C was the worse of them in my opinion however. The only time I got to Red, (since then my catridge kicked teh bucket) I had traded my starter for a Mr. Mime, which kicked ass back then, but that isn't the point. My whole team was near Red's in levels, I forget exactly what I had back then, with Mr. Mime being about five higher....and then Snorlax happened. I literally had to sit and PP stall him out of Rest, while watching my entire team go up in smoke. I won, might have also set a record for number of turns in an ingame battle.

IN HG/SS, Red was better, and didn't give me as much flack.

The DP era Elite Four are probably the hardest storyline trainers I have ever faced, alhtough that might be because I don't have the patience to grind in those games. Every time I clear Victory Road, my team is somewhere between 5 and 10 levels short of Cynthia's.

I just played through Diamond for the first time and I finally understand why people had trouble with Cynthia. The level leap is pretty crazy. You go from Barry's starter at level 53 to Cynthia's garchomp at level 66. Also I forgot to purchase Revives before heading in. Oops.

Fortunately at the time I was leveling up an RNG-ed dragon from my French game, so I ended up using him to DD and sweep the Elite Four, thanks to the 1.7x experience boost. My starter and OT pokemon basically did nothing.

Definitely Iris in Black 2. That beast of a haxorus kept killing everything.

It was the first time I had to go back and grind a lot more to be able to win (and abuse revives lol).

Brawly was the hardest trainer for me I had a level 19 zubat and a level 25 grovyle that just kept getting beat with his makuthas vital thrust move.

This here. It's frustrating to witness how bulky this thing is in-game.

On my 3rd or 4th playthrough, I used Excadrill to set up on Cofagrigous (3 Swords Dances + Sandstorm for speed). I regrettably forgot to teach it Brick Break or Xscissor, so I had to rely on rock slide.

The hit landed, but to my dismay, it survived. I just wonder if I had the worst Excadrill ever or something.

There is only one random NPC I have ever had trouble with in any pokemon game.
In Platinum on route 210 there are two Cooltrainers who engage you in double battle and holy shit I hate them.
Level 36 Gyardos with Aqua Tail and Ice Fang and a Level 36 Raichu with Thunderbolt.
OMG
I dont even remember why I lost but I have list to these two on different play throughs at least twice.
I think one of these was because it was that stupid Defog route and I didnt Defog so I got trolled by accuracy and I think maybe the seconds was when I had Empoleon and my team was under leveled.
I hate themmmmmmmmm

I'm doing another play through of Emerald and Wally in Victory Road was really tough. Gardevoirs future sight OHKOd most of my pokemon and my team was pretty psychic weak too. Wally can rip through you if unprepared and I was lol

Honestly, I can't remember ever having trouble with NPCs.

I guess the only exceptions to this was Red, but that changed in a hurry. I had trouble with Brock (I picked Charmander) and I remember grinding like crazy to get an Oddish up to a high enough level that it could fight Misty.

After those two experiences, I always kept a balanced team, on any game. Blue was the next one I played (I was 10 at the time) and my team was something like Venusaur, Raichu, Magmar, Cloyster, Golem, and Gengar.

The next game was Yellow, and my team was about the same in terms of type balancing. After that was Gold, and my team then was Typhlosion, Ampharos, Xatu, Quagsire, Sunflora, and Heracross.

The trend continued, and in Gold I learned to put most of the HMs onto other pokemon, and I taught myself toxic stall with Sunflora. The last lesson I learned about in-game runs was not to put anything on my team unless it was going to be there until the end. That way, EXP isn't wasted on things that I may not keep on my team, and my team's average level is higher, even if I only have 3 or 4 Pokemon for half the game.

I think I had some problems with NPC in pokemon Colosseum. Phenac Gym leader was a big problem to me, I hated it some much!! >:(
And to catch Suicune and Raikou from Venus and Ein?! What a pain!! xDD

I had a Modest Excadrill, I know exactly what you are saying. And that toxic-protect stall thing and 145 base defense is ridiculous

Two stand out for me.
Back in GS, there I was, strolling into Blackthorn Gym. "Dragons? But you only have Dragonair at this point, right?" I asked myself. Seeing Horsea, I made a connection to SeaDRA, but hey, that can't be bad.
Then the battle begins and Piloswine makes minced meat out of those Dragonairs, though it did sustain heavy damage. Out comes Kingdra, and I go "Oh, Seadra got an evolution? Cool."
Switch out for Lanturn
Thunderbolt
Watch as it doesnt even do a quarter of damage.
Get swept.

That hurt.

In HGSS, on the route to the Safari Zone, theres a double battle with a Magmar and an Electabuzz. They hax me every damn time, it shouldnt be THAT hard a battle!

That one always gets me as well

My bad, I need to clarify that.

Hydreigon was the pokemon I needed to take down. Cofagrigous wasn't a problem because Shadow Ball does little and 3 SD is enough for any Excadrill (Not to mention Toxic and Protect were free turns for me to set up).

But yeah, Hydreigon surviving +6 rock slide proved that my Excadrill was crap.

Earthquake / Magnitude took care of them

You probably don't have the Earthquake TM by that point and the only easily available thing you can have at the time is Geodude.

Whitney's Miltank and uhhhh...... Youngster Joey. What a beast Joey is.

Honestly, I have never a dreadful battle in any main stream pokemon games.
Pokemon Colosseum:
1st time Venus: Her attract and Suicune were extremely difficult, one of the few Pokemon trainers that I had to fight more then twice.
Pokemon XD:Gale of Darkness:
1st time Gorigan: I was young when I played this game so I did not know how to counter Protect-Earthquake. I believe it took me 10 attempts to beat him. I remember hating him with a passion. I actually had to go through this game twice because the next time I played after defeating Eldes, my save file got corrupted. The second time I fought him, it still took me about 5 tries to beat him.
Ardos: That shadow end Snorlax
3rd time Eldes: The Latios and Latias could sweep entire teams.
Pokemon Battle Revolution:
Master Battle set 8 Master Class Canata: the use of trick room and spore parasect.
Master Battle set 6 Sashay: Her pokemon were extremely bulky and difficult to overtake. This battle took me forever.
Master Battle set 7 Master Class Razma: A very difficult battle that can change at any minute.

I know what you feel about Venus, she's a b*tch :/. I surprisingly caught her Suicune very easily, however, her attract + paralysis + confusion strat is awful :(.
The first time I battled Ein, I was surprised by him, I didn't know I would encounter a boss in this particular room so I didn't heal my pokemon and my Ampharos was dead which was my best pokemon against his team. After lots of confusion/paralysis/crit hax and potions used, I managed to beat him and capture his Raikou but this was awful :/.

I'm playing XD right now and I found trainers with very original strategies that can be very effective :
- There's a dude in the tower colloseum that says that he likes fighting type pokemon. I thought : "well, that's not a problem, I have Gardevoir in my lead spot". He sent out Hariyama and Misdreavius ("WTF Misdreavius ?"), he used fake out against Gardevoir and Misdreavius used Imprison to prevent my pokemon from using psychic and aerial ace.

- There's a cypher grunt that leads with Wobbuffet and Shedinja which means that if you don't have anything that can hit Shedinja in your lead spot, you can't switch out unless you kill the Wobbuffet which is very risky. I luckily had leech seed on my Jumpluff to kill Shedinja. The funniest thing about it is that my Gardevoir traced Wonder Guard, I didn't know this ability could be traced, that's good to know :D.

- There's another trainer in the tower colosseum that leads with a Wobbuffet and a Misdreavius. His strategy is to use perish song and trap me with Wobbuffet. The funny thing about this is that my Gardevoir traced Shadow Tag so everyone on the field would die to perish song due to being trapped :D.

These trainers may not be particularly hard but at least they have very original strategies that could be deadly in some situations.

Oh, and speak of which, in Colosseum Doubles Mt. Battle (battle mode), on the road to Ho-Oh. These trainers are also in the Battle Now mode. My team was Ampharos, Entei, Suicune, Flygon, Espeon, and Umbreon. I had no problems with Desid, who ironically I find the toughest in Battle Now.

Platinum Cynthia. Luckily my Luxray had Ice Fang to beat Garchomp and my Lucario was JUST faster than hers.

In all honesty I read this entire thread and no one mentioned that PAIN that is Cooltrainer Jennifer on Hoenn Route 121? (Not in Emerald, as she has a Sableye)

Jennifer has a Level 31 Milotic in R/S, which probably is the hardest regular NPC pokémon because of:
- It's always raining in Route 121, which means you face a Water-type Pokémon with 100 base Special Attack whose STAB is boosted
- Only one STAB move, but it happens to be the ultra-annoying and haxy Water Pulse. Combined with the rain, you need your pokémon to stomach a 140 base power move with a 30% chance of confusion
- 95/125 special defenses mean that your Electric-type will not be a good choice. Even Grass-types are likely to have a hard time in this battle
- Status is not even an option unless you've taught one of your pokémon Toxic, as Milotic has Recover and can take advantage of status with Marvel Scale and her now-boosted defense (Remember that is not weak to physical moves in Gen III)

I lost THREE TIMES to this Milotic in my first playthrough. Granted, I started with Torchic but even on later playthroughs I lost two to three pokémon to her

I've sort of had problems dealing with Roxanne's Nosepass's ridiculous defenses, Brawly's Makuhita's Bulk Up and Wattson's Magneton (Tip: If it's your first time ever battling him, BRING A GROUND-TYPE) the first time I faced them, but I quickly got over them. Likewise for Tate and Liza in Emerald

Other than they, I've not had much problems with any other trainer other than the already mentioned ones (The Mt. Moon Rocket Grunt, Misty, Whitney, Morty -Not in HG/SS though, as I swept his team with a Level 15 Togepi thanks to Extrasensory-, Fantina in Platinum, Cynthia...)

Even then, I may not consider these trainers tough again after playing that challenging hackrom that is Pokémon Volt White 2... Burgh is HELL ITSELF in Challenge Mode (Really? Occa Berry Escavalier with Megahorn at the third gym!? Scald Masquerain? Accelgor with Focus Blast, Giga Drain and U-Turn? Victini is far from the best method to win) while Drayden is a serious juggernaut even though I beat him in the first try