By the power of truth, I, while living, have conquered the universe.
A single Forerunner fortress-class ship could wipe out the entire Reaper fleet, as the highest ranking Forerunner Warrior-Servant I'm going to go ahead and say that the Mantle's Approach would be a pretty lethal force against the Reapers.
It should be noted that Reapers are not that powerful by sci-if standards, it's just that Mass Effect tech is weak.
Plus he can just compose the Reapers into his own army.
[Edited on 12.06.2012 5:14 PM PST]