Please help me pick my third pokemon

Hi, this is my first time stepping into competitive pokemon battles so I need some help. My current goal is to reach 100 + Win Streaks in Battle Tree. I have two pokemon ready, need my final one for the team ;

Golisopod @ Assault Vest ??
Nature : Adamant ( Atk+ Spa- )
EVs : 116 Hp / 252 Atk / 44 Def / 96 SpD
~ First Impression
~ Aqua Jet
~ Leech Life
~ Bulk Up ( not sure about this one )

Toxapex @ Black Sludge
Ability : Merciless
Nature : Calm ( SpD+ Atk- ) / Sassy ( SpD+ Spe- )
EVs : 252 Hp / 128 Def / 128 SpD
~ Recover
~ Venoshock
~ Toxic
~ Scald ( not sure )

After four days of breeding for abilities and IVs I got tired of trying and settled with this Merciless Toxapex. My original idea was to go for a Regenerator but it was taking too long.
I need a third pokemon, I'm thinking about a Mimikyu that's bulky but does damage too.
I want to center my team around toxapex and want to have a Balanced / Bulky team
Thanks

Bulk Up can't be used with Assault Vest, so you'd either have to drop the vest or replace Bulk Up with another move. Liquidation or Sucker Punch are probably good options, and there may be other attacks worth using besides.

But as for your 3rd slot, what you might need most is some sort of Electric resist, as both Golisopod and Toxapex are weak to Electric. A Ground type would probably be best, as they're straight up immune to Electric and Golisopod/Toxapex more or less handle the Water and Ice weaknesses. Landorus (either form) would be good if you have a suitable one. But if not, you have other choices too.