I have him at 5*, 0/85 shards. I farmed pretty hard to get extra character shards of my current 7* lineup but it's slow and in my opinion it's not really worth it at the moment. I decided to hold off until he's a regular farmable character in the Arena or Cantina. I encountered him once in Arena where someone had him at 7* maxed out at lvl 70. Grievous didn't do much damage and his taunting was blocked by my jedi team. That's when I stopped being impressed and put him in reserves.