Toxapex can be 2HKOed or OHKOed by tons of pokemon. You could try something that sets up a substitute that Toxapex can't break, maybe a Buzzwole or Zygarde 50 and proceed to kill it or set up on it.
>I use taunt but they can just switch out
Forcing switches is good. Try getting some entry hazards up, or predicting their switch-in to your Pokemon and sending out a new one that threatens in out. A large piece of battling is having the advantage and forcing switches frequently.