Name: Magby
Availability: Early. Found in the Virbank Complex before the second gym.
Stats: The Magby line has above average Attack, Special Attack, and Speed, but Magby also has mediocre defenses, particularly on the physical side.
Typing: Fire is an acceptable offensive type, and the coverage against Steel-types in particular is appreciated. Rock, Ground, and Water weaknesses aren't helpful given Magby's frailty so you must ensure you use Magby with something that can take these moves.
Movepool: Its level-up movepool is limited to STAB and Faint Attack. Flame Burst at level 22 is nice for early power, and Lava Plume and Flamethrower replace it later. Magmortar learns some cool moves, although Thunderbolt arrives too late to see a lot of use. ThunderPunch from the move relearner can be used in the meantime.
Major Battles: Does well against Roxie and Burgh, and is very good at dealing with Colress, the Shadow Trio, and Zinzolin. It deals with Skyla's Swoobat and Skarmory nicely, but most of the other gym leaders are strong against Fire and there aren't many Elite 4 Pokémon that stick out as being countered by Magmar/Magmortar.
Additional Comments: Magby's main problem comes from late evolutions. It lags a bit just before level 30 and it would have likes to evolved into Magmar sooner. Magmortar is very good but Magmarizer comes too late for Magmortar to have much of an effect.
Name: Audino
Availability: Found commonly in shaking grass on almost every route. Note that you need to get the first badge before shaking grass starts appearing.
Stats: Audino's Speed and attack stats are at least on par with most of the unevolved Pokémon at the start of the game, but they lag late-game. Audino's strengths lie in its nice 103/86/86 bulk.
Typing: Pure Normal is a bland typing with a Ghost-type immunity, a Fighting-type weakness, and no super-effective coverage. It does have good neutral coverage though, which is ideal for taking on consecutive trainers on routes. Ghost-, Steel-, and Rock- types don't care about Normal-type moves but Audino can cover them with other moves.
Movepool: Very wide. It has plenty of STAB options to pick from, with Return being available near the start. Coverage moves include Ice Beam/Blizzard/Ice Punch, Surf, Thunderbolt/Thunder/ThunderPunch/Wild Charge, Flamethrower/Fire Blast/Fire Punch, Psychic, Shadow Ball, and Signal Beam. You can mix and match moves to suit your team.
Major Battles: Mostly depends on what moves you teach it, but late-game it'll find that it's too weak to sweep teams. Basically just pick match-ups that suit its moves.
Additional Comments: Healer is very situational and Regenerator is definitely the preferred ability. Audino tend to pile on damage late-game, so any free healing you can get is appreciated.
Name: Shuckle
Availability: Found on the bottom floor of the Seaside Cave with a 5% encounter rate.
Stats: Shuckle's Attack, Special Attack, and Speed are laughably poor. It has options to boost its Attack, but its low Speed and reliance of Attack-boosting moves will prevent it from sweeping opponents effortlessly. On the other hand, Shuckle has the highest defenses in the game, although its low HP limits the amount of hits it can take somewhat.
Typing: Rock/Bug is a decent offensive typing with coverage on Flying, Grass, Fire, Psychic, Dark, and Bug. Defensively it isn't so good, with just 2 resistances, although Shuckle can survive anything regardless.
Movepool: Shuckle has a few moves to boost its lousy Attack: Power Trick (massive Attack boost, but is physically frail and still slow); Shell Smash (Attack/Speed boost, but needs many boosts and becomes frail); and Power Split (also weakens your opponent, but comes later than the others). It gets Rock Slide and Bug Bite from level up, and Bulldoze from TM for coverage against Steel-types.
Major Battles: Shuckle will never be the best option for a match-up, mostly because its Speed is so low. Power Split should allow Shuckle to do well against Caitlin, although its low Speed will make a sweep difficult.
Additional Comments: Sturdy is the better ability, although it won't affect Shuckle's performance significantly (so don't worry if you get unlucky and encounter a Gluttony Shuckle).
Reserving Petilil, Lapras, and Patrat.