He is required for all intents and purposes. GK + Nihilus may be more stable in arena...but....if you have those two, just go for the full meta team and ditch resistance altogether. I can't imagine resistance without RT...90% TM gain if he pops an expose and lands a new expose on basic...amazing way to recover when you are otherwise on a bad run of RNG.
For HAAT he is not mandatory unless you want to deal a ton of damage. Still, you can technically enter battle without him, so I can't say he is mandatory.
Also...not to pick on you....but a few seconds googling before farming resistance might have been a good idea.
