This crazy thing is slow but it actually has a great movepool.
Stunfisk @ Choice Specs
252 Sp.Atk, 252 Sp.Def, 4 HP
-Thunderbolt / Thunder
Straightforward. Thunderbolt and Earth Power are STAB, Surf deals with Ground types while Sludge Wave deals with Grass types. Thunder is an option over Thunderbolt if you have rain support, as this crazy thing (what is it actually) also gets surf it can use rain well.
Limber is completely useless as it is a ground type so already immune to thunder wave. Also, Stun Spore or Glare are pretty rare and if they still happen they won't bother Stunfisk too much because it's already almost as slow as Steelix.
Master balls are the cheapest thing ever invented. It's more fun catching everything with balls that can fail. 'caught Latios in the wild without master ball ftw'