heh, my memory is foggy. Did TIM ever mention if the gun that disabled that reaper we board was a space-based weapon? or land-based? If it was aboard a ship, well...I doubt that thing even exists anymore. But if it was land-based, perhaps our galaxy could find it and copy the design. Something big enough to drive a canyon through a planet with a glancing blow and still have enough power to disable a reaper

Can I haz?
But really, it would take multiple superweapons to kill the Reapers. Aren't there Hundreds of thousands of them coming? (maybe I'm wrong, might only be thousands.) Even with the combined power of the entire galaxy, it would be bloody hard to pull off, considering how incredibly tough the Reapers are built.
Sure, we killed Sovereign (with the help of an entire fleet.) And a quasi-hybrid-thingamabob Reaper. (Which wasn't finished, so that's not really a kill.) But these other ones aren't going to be by themselves, they'll have dozens or hundreds of buddy Reapers to keep the pesky humans off their backs.
I can see them falling for deceptive tricks, however. Like someone else mentioned, luring them into a starsystem and then blowing the Star to kingdom come. They haven't fought a war against someone who could effectively fight back for hundreds of thousands of years, they're going to have to brush up on their warfare tactics to deal with that, it could buy the galaxy some time and some kills from arrogant reapers.