The point I'd like to add is: adaptation.
The Reapers as it turned out, had been carefully guiding the development path of most, if not all, sentient space faring species of Mass Effect universe. This is how Reapers themselves are perfectly adapted to the role they assigned to themselves: to destroy eg 'harvest' all those species who have grown so far thanks to their silent guidance.
The analogy is perhaps, if someone has been studying jiu jitsu, thinking he's done so well, only to meet in the end an enemy who actually created jiu jitsu in first place and thus not only know perfectly how it works but also know perfectly how to beat it. Hmm sorry if jiu jitsu seems like poor analogy, but that's the idea.
This is how Reapers seem able to rather easily kill the space faring species like Proteans, Asari, Humans, etc, because they already know what toy guns they expect from us since they're the ones who gave us those toys in first place.
But an encounter with Tresher Maw is something that is probably outside their expectation. The Tresher Maw is a powerful creature that had evolved totally independently so Reapers have little idea about them. The Tresher Maw is perfectly adapted especially to the Krogan homeworld of Tuchanka so that's home ground advantage. And in that particular scene, the Tresher Maw totally took the Reaper by surprise, approaching with an attack that that Reaper had little idea how to defend against since it never anticipated it.
So at that particular unfortunate encounter, a Tresher Maw killing a Reaper is actually very believable.
Of course a surprise attack may likely only work... once.