To gain this achievement you must complete Grunt's loyalty mission ("Grunt: Rite of Passage"). His mission involves returning to Tuchanka and facing a series of battles.
Like all the loyalty missions, Yeoman Chambers will eventually tell you that Grunt wishes to speak with you. At this point, head down and talk to him. After this conversation, head to Tuchanka and bring Grunt with you to speak to the current leader of Clan Udrnot (in my case, it was Wrex). The clan leader will direct you to speak to the clan Shaman about the rite of passage, do so, and he will set you off to the actual mission.
During the combat portion of the mission, you'll be forced to defend yourself against three waves of enemies.
The first wave is against Varren
The second is against the fire-breathing Klixen.
The final wave is against a Thresher Maw. You don't necessarily need to kill the Thresher Maw, but doing so will net you
If you really want to defeat the Thresher Maw, I would suggest bringing the M-920 Cain with you as it will pack a huge punch against it.
After surviving against the Thresher Maw, the final test is defeating Gatatog Uvenk and his krantt (krantt = group of Krogan). Do so, and the achievement is yours.