The lone Thresher Maw in Mass Effect 2 is faced in Grunt's loyalty mission "Grunt: Rite of Passage", after clearing the last wave of regular enemies, there will be a rumbling and the Thresher Maw will raise from the ground and you will have five minutes in which to defeat it.
The Thresher is a powerful and dangerous enemy especially on higher difficulties, but the strategy isn't too hard. The Thresher has a single Armor bar for both health and protection so ammo powers like Incendiary, Warp and Armor Piercing ammo are recommended if your Shepard or your other Squadmate has access to. Any other power that is effective against armor like incinerate, warp, reave and others also work very well in dealing large amounts of damage, any kind of Heavy Weapon can also quickly rack up damage as well.
Since Grunt is required for this mission, make sure he has incendiary ammo turned on, even though the power wheel will show it as ineffective, Concussive shot does stagger the Thresher and prevents it from launching as many acid barrages at once. Garrus Zaeed and Solider Shepard can use this strategy too if present to chain stagger the Thresher to help avoid damage.
Shooting the blue tendrils near the Threshers mouth deals significantly more damage than the rest of its body, but these tendrils are somewhat difficult to hit.
As long as you keep a good amount of pressure on the target you should defeat it with over a minute remaining even on Insanity difficulty.