Yes. 6* for arena. IDk about CB or Scarab though

Without knowing the specific multipliers on his abilities, it's hard to answer. But one thing is clear - he's pretty expensive to book. 13 tomes is definitely on the high end for a lego, and you'll 100% want to get his A2 and A3 fully booked to get the reduced cooldowns.

As for his usability, I think he would be very solid as a dungeon boss killer. But for CB, unfortunately, I think his A1 makes him much less useful. He won't fit into any unkillable or counterattack team cleanly, as his A1 will mess up your turn order. On unkillable, that'll probably mean he ends up dying because his unkillable drops off before it can be renewed, while on CA it means he'll go out of sync with your CA rotation. If you're running block debuffs to prevent the stun, there's a good chance he'll fall out of sync with that too, and therefore start absorbing stuns.

I really, really wish they didn't add that part in. If the 15% TM boost against bosses were removed, he'd be a very solid nuker on CB teams.

I belive Ninja has been thought for early game. Usefull without books, just for beginners, to go with your starter champion at lvl 50 (2 lvl 50 for the effort of 1... is a cool start).

Helping new people to defeat first dugeon´s stages, beat CB in "Easy", "Normal" and "Hard" difficult.

His A1 prevent to use him in late stages because he cant be used in inmortal/ca setups (at least in a first fast look to his ability).