Faust starts out very powerful and is nearly impossible to take down quickly.
The minions will be filling the soul wells and you are suppose to prevent them
from doing so, but really no matter what you do, Faust will get enough soul
wells to consume them.
There ends up being a couple of ways you can go through this fight. You can choose to ignore everything that is going on and just focus on damaging Faust. To do this, just target Faust and toss out occasional AoEs for the adds that always tend to get in the way. Another route is to play Faust's little game, defeat the minions, avoid Faust's soul consuming hissy fit, and then pound him with everything that you have once he's drained of power. You may end up having to do this through a couple of waves, but avoiding his hissy fit will cause you less hurt. Probably a combination of the two is best. Push light damage onto Faust while cleaning up the minions then go nuts when he's drained. It might take longer but if you're having trouble surviving the fight, it might work better than the first method.