Edit: I'm bored and going though some solutions. This one was pretty outdated and needed a revision, so on the dawn of Mass Effect 3 I present the newer solution.
So, the Thresher Maw was a pretty intimidating enemy in the first game, randomly appearing on uncharted worlds; making short work of the Mako. Not anymore, though.
There is only one Thresher Maw and you shall be fighting it on foot. You will encounter it by doing Grunt's loyalty mission (note: If you do not release him from his tank, this achievement cannot be unlocked. I am unsure if releasing him after going through the Omega-4 Relay will still allow you to access his loyalty mission or not).
Fighting it on foot is pretty easy, depending on the difficulty you are playing on. You are given 5 minutes to "survive" it, but it should take less than half of that amount of time to kill it. It is you, Grunt, and one character of your choosing. It's health bar is strictly Armor (even on Insanity). The following weapons and skills will prove useful:
-Almost any heavy weapon, Heavy Pistols, Sniper Rifles, some Assault Rifles
-Incendiary Ammo (Vanguards, Soldiers, Grunt, Jacob)
-Warp (Adepts, Sentinels, Thane, Miranda)
-Inferno Grenade (Zaeed's loyalty power)
-Warp Ammo (Jack's loyalty power)
-Armor-Piercing Ammo (Garrus' loyalty power)
-Incinerate (Engineers, Infiltrators, Mordin)
-Reave (Samara's loyalty power)
The Thresher Maw doesn't require any super-crazy tactics. It will shoot the lovely acidic saliva at you causing a fair amount of damage to you. You can use the pillars littered throughout the battlefield for cover, but they can be destroyed by the Thresher Maw. It never gets close enough to attack you with melee strikes, but it will circle around the arena.
The fight isn't very difficult (especially on Normal or Casual), so this is an easy 10 gamerscore