Well, this brings us back to Spellcasting Harrier, which adds half your level to the Concentration check (IIRC).Mercule said:I loathe the absolute nature of the feat. It runs totally counter to the "you can try anything" nature that 3.x was supposed to possess. Plus, it's just plain annoying.
It would be much better to have it be a modifier to the threatened caster's Concentration check. Either something like a flat +10 DC or change the DC to (15 + 1/2 aggressor's level + spell level) or change the DC to (10 + aggressor's highest attack bonus + spell level). I'm not sure those are the best numbers, but those are the sort of concepts I'd rather see.