I'm actually having the opposite experience. Corso would consistently die on me halfway through a fight UNTIL I hit 30 or so. Now he's halfway decent. I keep giving him gear upgrades whenever I can, maybe that's part of it. My gunslinger is 37 now and while Bowdaar still fails me because he isn't well geared, I can count on Corso to at least do (and take) enough damage to keep me alive.