Omastar @ Leftovers / Shuca Berry
Ability: Shell Armor
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
- Scald
- Ice Beam / Icy Wind
- Toxic Spikes / Spikes
- Stealth Rock
Steelix+Sliggoo is something a lot people like as a pair but i honestly prefer these two just due to the amount of things Omastar can do for Sliggoo that Steelix can't
Typing - Omastars nice Rock/Water typing gives it an advantage over steelix in various ways such as taking resisting strong physical attacking moves like Normal,Poison,Flying and also resisting Ice which helps out against Jynx. However it also helps with that Omastar is threatened by with common weaknesses to Electric+Grass this invites in a lot of checks towards Omastar which gives Sliggoo an easy chance to get in but it also keeps its status of being checked by fire types due to the coverage most have on hand in HP Grass/Thunderbolt.
Hazard Game - This is especially why i prefer this thing the fact it gets all the damaging hazards lets you fit both mons on Hazard-Stack,Balance,Bulky Offense and even Semi-Stall thanks to Stealth Rock+Spikes/Toxic Spikes.
Other aspects such as scald burns are nice but the other two reasons are just so awesome from my time with these two