The amount of damage you have to do is either 20%, 25% or 30%. It's in that range, at any rate, and I say this as a caster who got frustrated trying to tag FATE mobs when everyone else was a pugilist or gladiator. In the 2.5 seconds it takes me to nuke a mob, so many players have already hit it, that getting the second cast necessary for credit wasn't possible.

So it's definitely in the 20% - 30% range, but not higher from what I saw, and not lower.

You do get full experience, and quest drops. Normal item drops, I'm not so sure.