Worth Training up Arvis?

Yes, he is very powerful, and can debuff a lot of enemies.

If possible use allies that boost his speed and use a water blessing.

Not if your building your best Arena team(lack of merges)
Yes if you will be using him for anything else

yes, he's pretty much almost ready out of the box

just give him ardent sacrifice, fury, and iceberg and he'll serve you well.

if you have a spare deirdre you can give him spd ploy and he can run all 4 ploys

Yes, Arvis is a fantastic unit. His ploy debuffs and ability to pseudo-heal is very helpful, especially in TT from my experience. Pretty decent in Arena from my experience as well.

He's a ploy boy, and ploy boys are the best

i spent 20k onto LYON, and i regreted.... but with arvis was different...
i liked him so much.
a character that could benefit form fury without recoil damage? with free "mend"? if you put seal of + 3 spd, he is a good mage, as for tanking anothers mages.