Legendary difficulty

Yes sir, as you wish sir.
C3PO (due to the fact that not many people work on Ewoks in the game)
Chewbacca
Thrawn (thrawn is just so hard to go against anyways no matter where he is being fought)
BB-8
R2-D2
Palpatine
Yoda (super easy)

Chewie - Literal hell, stupidly difficult regardless of the leader
3po - Hell but at least you have a revive to offset bad rng
Thrawn - Tough, doable with and arena mods
R2 - doable but without good empire toons it will be tough, very tricky if you don't have at least 1 legendary toon for a good lead (unless you have another good empire squad Ie: Imp troops geared up)
Bb-8 - was hard but kru and Fox made it much easier,even without zetas
Palp - Okay rebel toons can do the trick, g8 Phoenix can just walk through the event due to the synergies
Gmy - You can clear this event with so many jedi it's stupid

Wow, this explanation is far better than mine. Go with his.

Anhalds Chewbacca Video is hilarious

In my case thrawn was easy, because i like Phoenix

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.