Toxapex should never stay in on Tapu Fini. If Tapu Fini happens to use whirlpool, Toxapex is guranteed to die. Whirlpool Tapu Fini isn't common yet, but it tears stall to pieces, so defensive mons need to beware.
Even if it doesn't have whirlpool, Toxapex is still not going to get anything done...