First of all, the point in being able to switch builds is to adapt to situations. A single target dps build will rock an aoe build when you face that boss without adds.

Second; Guys & Gal's, seriously. It's not so much about your build as it is about your gear. Most builds are viable to some extent. Get some damage gear, else you'll have trouble NO MATTER what build you run.

Also, soul harvest can increase your damage by a ton. I'm talking about going from 900dps pre SH to 1200 with SH. Friendly tip: Use it to your advantage. Friendly tip #2: It's not to your advantage on a single target dps boss.