Crustle

Crustle doesn't have much going for it. It has two of the worst defensive typings smashed together, and they don't have much synergy to boot. Its attack is too low to be considered on offense, and its defense and easily abused typing leaves it undesirable for defense. If you really want to use it, its Bug Type moveset is more effective than its Rock Type moveset, but only because it has less competition in that regard. Its one real advantage is the fact that it has a giant rock-lasagna on its back, and that advantage should not be underestimated!

Best PvE Offensive Moveset

• Smack Down and Rock Slide give Crustle a STAB Rock Type moveset
• Fury Cutter and X-Scissor give Crustle a STAB Bug Type moveset
• Rock Blast is a weaker option overall.

Best PvE Defensive Moveset

• Smack Down is leagues better than Fury Cutter on defense
• X-Scissor and Rock Blast both have the advantage of being 3-bar. X-Scissor is off-type with Smack Down and is harder to dodge, but Rock Blast has higher damage and more relevant coverage. 
• Rock Slide hits harder but less often.

Furry Cutter and Smack Down are polar opposites of each other; the former focuses on fast energy generation at the cost of stronger damage output, while the latter forgoes energy gains in favor of more STAB damage. Either option can be viable depending on the situation. 

X-Scissor costs the least amount of energy, and provides decent Bug-type coverage backed by STAB. Rock Slide is a very efficient charge move, costing a moderate amount of energy with a very respectable 1.8 DPE (Damage Per Energy). Rock Blast is inferior.