RE: Hulking Hurler
1. The hulking hurler's damage scales with his carry capacity
2. Carry capacity scales with strength. Every +10 strength = 4 x carry capacity = 4 x damage
3. Carry capacity scales with size. Each size increase doubles your damage
4. Carry capacity increases if you've got more legs - quadrupeds carry 50% more. Being a quadruped increases your damage by 50%.
Centaur + hulking hurler + strength boosts
Strength 36 (not unreasonable). Carry capacity 7200 pounds. Damage? 39d6. If it's a spiky ball, that's 78d6.
Each round - because he enchants his huge ball of lead with returning.
Beyond that, try some war hulk levels. +20 strength (10 levels of warhulk) is x16 damage.
Or start with something that's still humanoid (like a half-ogre) and use enlarge person on it?
For full details, see
here where some others work on the same thing. I think the planet gets destroyed on about page 3.
My suggestion on vetting PrC's?
DON'T!
All you have to do is this:
When someone takes a PrC, or shows you a character build, ask them why they are doing things how they're doing them. Ask them what the character is intended to do.
Chances are if they are a powergamer, they know exactly where they're going and exactly what the build can do. If you're an honest and good DM, then they'll tell you. Then if you find it to be off-planet, you can discuss what to do about it.
If they're not a powergamer, then the chances of them stumbling upon something truely game-breaking are slim, and when they do, they're likely to see reason.