If you have the Strength confidant high enough you can use the lockdown option to get null skills for any persona's weakness, throw grimmy in there for a few days and he'll gain the null bless skill, if that's his only weakness. I forget. Otherwise your best bet is sacrificing something else with null/absorb/repel bless into him.

hm ok dumb question asinine really but how do i up their arcana exactly do i just do the requests or what?

Pretty much yeah, the twins' SL goes up with every so many requests you complete.

Fuse the Personas they ask for, that's it.