Start Galvantula and Slaking, make sure Galvantula is Timid with 4HP + 252 SpA + 252 Spe.
Use Gastro Acid as your first attack, you need to boost Slaking immediately, no matter what.
If they go for Slaking first, Galvantula can Sticky Web to set up the rest of your team.
If they go for...