✅ Well I'm a huge Mace fan, so I would never underrate him. But I'm being realistic. In a saber-duel, yeah, Mace would last for a bit against Exar, probably longer than you said. But with force powers in play, Mace'll fall. This thread should be no Force Powers, so Mace and Palpatine would at least be able to put up a good fight.
Well, let's assume Exar was forsaking his saber powers for the chance to whup some ass. Mace is very good for his time, and he quickly vanquished Sidious in their fight. Sidious, however, is quick to get himself put on the defensive, and if someone like Mace Windu can pwn him, Exar Kun should be able to flay him alive.