Mass Effect: Legendary Edition (PC) - Achievement - Big Game Hunter
ME2: Thresher Maw defeated
*SPOILER ALERT*
This achievement is obtained during Grunts loyalty mission in Mass Effect 2. A prerequisite is that you have tried to recruit Warlord Okeer and instead found Grunts tank. You must have also have freed Grunt.
Essentially, he goes through Krogan puberty and you end up having to survive waves of enemies with the final enemy being a Thresher Maw. All you have to do is survive, but to get the achievement, you must kill the Thresher Maw. It is heavily armoured, so it's best to use incendiary attacks. This is also a good opportunity to use the 'Cain' heavy weapon which is basically a mini nuclear launcher. Using it in close quarters will kill you as well, but because you are outside with plenty of distance between you and the Thresher Maw, there isn't much risk of this and a 2 hits with it should kill it.