You killed Belial with 31k dps? You must be playing a class that can survive multiple strikes from invisible naga.
My demon hunter cleared all of act 2 using nothing but drops I found - I never set foot in the auction house. Non-sharpshooter DPS was under 30k. My monk cleared act 2 while spending under 500k, and none of that was spent on any ilvl 63 loot.* My friend's witch doctor killed Belial with around 30k DPS.
Also, there's no invisible naga during the Belial fight.
*It's easy to recognize ilvl 63 loot if you know the base item types, and I purposefully avoided buying any items above ilvl 62 because it felt like cheating.