5 Item levels are reason enough that he should be doing more DPS than you.
You used only one Tol'vir Potion and you're not hitcapped.
Don't start with Serpent Sting, you'll want to save your focus for the first phase until the boss is under 90% health. Use Aimed Shot and Steady Shot, then apply the dot and use Chimera shot. The other Hunter manged to use Rapid Fire two times in the fight, you only used it once.