Based on your knowledge of most fictional creatures/beings, who would be the most challenging but ultimately defeatable enemy Kalros the mother of all thresher maws could defeat?
Kalros is the largest Thresher Maw seen in the Mass Effect universe to date. It took down a land walker reaper all by itself, crushing and dragging the miserable thing into the ground. To call Kalros Colossal would be a understatement. Kalros can tunnel through solid stone like we do sand, it practically swims through the Tuchanka. The Krogan don't even think anyone will ever be able to kill it, they have erect murals and a entire arena dedicated to this seemingly unstoppable force of nature.