As mentioned before, Octillery has a massive Special Attack movepool, along with a high Sp. Atk stat. It can learn special moves from almost all types (it can learn Flamethrower, Sludge Bomb and Energy Ball while being a monotype). The only flaw to it is its fairly low Speed stat, and its mediocre defensive stats.
However, it can still be extremely useful as a sweeper:
Nature: Quiet (or less likely Modest)
Ability: Moody (Sniper if you're playing unofficial metagames)
EVs: 252 HP/252 Sp. Atk/4 Def. or Sp. Def
IVs: 0 Speed if possible
Item: Assault Vest (Scope Lens if you have Sniper, Quick Claw if you want to take risks)
-Water SPOUT (not to be confused with Water SPORT). I'm surprised that this wasn't mentioned already. It is an extremely powerful move that very few can learn. It also hits everything on your opponent's side of the battlefield, so it's less riskier than Surf, however if you lose a good fraction og your HP, this move's power decreases pretty badly.
-Flamethrower, Ice Beam, Energy Ball, Sludge Bomb, Psychic, Signal Beam, you name it!
-If you have Sniper and Scope Lens, you might want to try using Focus Energy, but it could potentially waste a turn to give your opponent the advantage over Octillery's defense stats. Octillery likely won't faint on the first hit, but it can definitely happen.
Adding on to this, if you purposely give Octillery low Speed stats, you could easily make a great Trick Room sweeper out of it; you just need something else to use Trick Room before it. I would highly not suggest sending Octillery out first for this reason.
Also, as for the ability Moody, (it boosts any random stat - including evasion and accuracy - by 2, and decreases another stat by 1 at the end of each turn), I would suggest it if possible, as it practically gives you freebie stat boosts without wasting a turn (but it can lower the stats you need too, but not as often). However, due to the fact that it boosts evasion, it is banned on most online metagames, such as Pokèmon Showdown or Pokémon Online. If that's what you're using, I'd suggest Sniper as an ability. But if you are playing competitively on the official Nintendo games, the ability is not banned.