I presume immune would mean can't be chosen as the target of events, and assassinate definitely chooses targets.
I'm actually backtracking on my evaluation for this guy a lot. He can only make himself immune, and if he does, he'll most likely just be a 1/2, and if you buy him dynasty phase, your opponent sees it and can always choose to bid one. I think he'd only be good as a conflict character. Unless your goal is to punish your opponent for drawing cards, he's pretty mediocre. Good way to stay ahead on cards though, I guess. I wonder if it's best to just play him and bid 4 or 5 regardless. Intimidate your opponent into bidding low, then punish him on the card draw. Go for 4 so that if they do go full five and decide to just eat the extra +2 pol, you can still get most of your hand.