Toxapex

Right off the bat, Toxapex is not the attacker that you're looking for. It's far too weak offensively to be worth considering, so please look elsewhere. On the other hand, it's pretty decent as a Gym Defender. It's not going to KO many attackers, and its Poison typing is a double-edged sword due to its resistance to a number of types including Fighting and weakness to Psychic and Ground, but it should generally act as a decent speed-bump that can sit alongside the plethora of Fighting-weak Pokemon that tend to litter Gym Defense.

Best PvE Offensive Moveset

• Poison Jab and Gunk Shot make for the best moveset that Toxapex can use offensively.
• Bite, Brine, and Sludge-Wave are sub-optimal.

Best PvE Defensive Moveset

• Poison Jab has higher base power and STAB, but Poison is a fairly widely resisted type that rarely does Super Effective damage.
• Bite is weaker and generates energy a bit slower, but Dark offers counter-coverage as well as outstanding neutral coverage that keeps it about on a par with Poison Jab in practice.
• Brine is Toxapex' only multi-bar charged move, making it the go-to on defense.
• Gunk Shot is superior to Sludge Wave, but both are a bit iffy in general.

Poison Jab's strong DPT and EPT outclass Bite's, and gives Toxapex plenty of opportunity to farm enemies down.

Brine is Toxapex's only Water-type Attack and offers coverage against Steel- and Ground-type Pokemon, although it is not particularly strong. Sludge Wave is cheaper than Gunk Shot and sees more consistent use.