You will face this enemy in Act IV. This boss shouldn't be a big deal. You will meet him at the final of the arena. If you have problems with this boss you can always change your difficulty. Train your skills is also a good option. It's not a minigame, just a random fight against a littlebit stronger boss.
There are no special medals related for this boss, so you can just win this one on the easyest difficulty if you like.
If you defeat this enemy you can progress your way further through the game. You will enter the final Act V then.