Sages can be DPS but they're not that strong unless properly geared. They also lack AoE skills so everything is done 1-on-1. You may have access to Tornado but Sages are mostly Physical based.
You have Aura: Singularity, which drains HP/MP over a certain time. It's decent can't compete against other AoE skills that are based off PATK/MATK in the formula.