Legendary difficulty

R2D2's event is hilariously easy if you have Palpatine, and actually quite challenging if you don't. His passive bangs up Rebels for a big chunk of health every time they take a turn, and does double damage on Old Ben because he's both a Rebel and a Jedi. So their team has lost a ton of health before you even take your first turn.

Without him, I'd put it on par with Thrawn for difficulty, possibly a bit higher even depending on what team you're using. R2's constant stealth-spam and monster health pool make him an absolute chore to work around.