On Tuchanka in Mass Effect 3 we get to see
a thresher maw take down a Reaper
In Mass Effect 2 your small squad is able to defeat one during a quest.
If the thing that takes down a reaper can be killed why are the Reapers causing the galaxy so much distress and are seemingly invulnerable?
The thresher maw that ate the reaper on Tuchanka was considered the "mother of all thresher maws" by the Krogans. Whether or not it actually is the source of all other thresher maws in the galaxy is debatable, but one thing that this 'maw is is HUGE. While the thresher maw you fight in Mass Effect 2 is big (or probably the same size as the ones seen in Mass Effect), this "mother" was larger than the Destroyer Reaper it took down, which in turn was much larger than the 'maw you fought in ME2.
Also note that the Mother 'Maw physically grappled with the Reaper, pulling it to the ground and then under the sands. When fighting Reapers with ships the weapons used are all solid projectiles or molten metals propelled at near light speeds with very high associated energies. While a Reaper's kinetic barriers are able to easily defend against these high energy attacks they probably aren't optimized to withstand a (relatively) low energy wrestling match.
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.
First off, the type of Reaper slain was a destroyer class. Much smaller than the big 2-km long True Reapers in the class of Sovereign or Harbinger. Secondly, Kalross is the "mother of all thresher maws" AND is easily 20 or 30 times the size of a normal TM. If it had been a Harbinger-class Reaper Kalross would have probably been squashed like an ant under Wrex's boot heel. Basically it's a size dynamics moment where Kalross was just the right size to destroy it
Firstly, consider that a Thresher Maw is pretty much the single hardest biological thing in the Mass Effect universe. They are tricky to deal with using the tank mounted weaponry on the Mako in ME1, and when
you, Grunt and another kill one in Mass Effect 2, this is considered exceptional even by the standards of hard as nails Krogan
Of course this doesn't explain why one could take down a Reaper, which is generally presented as in a different league entirely, but presumably not all Reapers are created equal.
The logical explanation is that the Reaper was (a) weak (b) really unlucky. Remember that if a Thesher Maw emerged from under the Mako in ME1, it was game over, and in ME2 the Thresher Maw can't actually get that close, presumably because of the rock floor of the proving ground. Thus if a very powerful Thresher Maw caught a weak Reaper by surprise at close range, it may well be quite possible to kill it. While Shepard had much more trouble killing Sovereign, I think it's safe to assume that Sovereign was one of the more powerful Reapers.
The Reapers didn't simply nuke the planet because they needed humans for materials in building a new Reaper.
But to answer the original question it was a mix of a surprise attack and luck (for the humans) also the fact that the Reaper’s shields were meant to withstand projectiles.
Like the geth shields in Mass Effect 1 they were meant to withstand projectiles and so running them over with the mako does no damage to their shields but usually one shots them because the shields cant defend against a ton of metal ramming it.
Just as the Reaper’s shields were not designed to withstand multiple tons of thresher maw.
because that is a big plot hole. the reapers were made to be this huge threat yet they die to a couple of missiles and have to take Earth by blowing up 1 building at a time when a real threat would have just nuked the whole planet and be done with it.
