Gen IV Ingame
Cloyster (M) @ Leftovers / Whatever you have
Trait: Skill Link / Shell Armor
EVs: 4 HP / 252 Atk / 252 SDef or 4 HP / 252 SAtk / 252 SDef
Careful Nature (+SDef, -SAtk) / Calm Nature ( +SDef, -Atk )
- Rock Blast / Mud Shot / Whatever HP you can get with decent power
- Poison Jab / Surf / Whirlpool / Brine
- Spike Cannon / Signal Beam / Whirlpool
- Avalanche / Icicle Spear / Ice Shard / Ice Beam
Looks a little messy, but here's how it works.
You can go physical with a Careful nature, and the first EV spread, Making use of Skill Link. Rock Blast is still beastly back in Gen IV, so you can work with that. Poison Jab is a great way to have a main attacking move ( You need those ingame ) That you can drain your PP on, Spike Cannon gives you great neutral hits on Those annoying Trainer battles, and Avalanch, Icicle Spear, or Ice Shard gives you whatever Ice coverage you want for him.
Special set uses either ability, although the second one is prefered, Calm Nature, and the second Ev spread. Mud Shot and HP [Anything] are your options for your first slot, not too bad. Surf is hard hitting STAB and that much needed transportation, although in HG / SS you might want Whirlpool if you already have a surf user, and Brine can be used for great strategies, but isn't the best choice might I add. Signal Beam is gotten from a move tutor, but if you can get it, it works well. While Whirlpool is there again if you want a surfer and a Whirlpooler. Last slot is Ice Beam, which is a stand alone boss move for ingame and competitive.