Thresher Maw (enemy)

Mass Effect
Thresher Maws in Mass Effect are encountered on a number of uncharted worlds and can be fought in the Mako or on foot.

Tactics
You get experience twice when killing a thresher maw - once when its severely injured and once again when it dies. It is unknown whether this is a bug or intended behaviour. In consequence, when playing on hardcore difficulty, do not mistake the first experience reward for an indication that the fight is over, or you might be in for a nasty surprise.

Unlike other enemies in the game, such as Geth Armatures, thresher maws do not have to be even close to looking in your direction to perform any of their attacks, so do not attempt to drive into or melee attack one.

In the Mako

 * When driving through thresher maw nests, be on your guard. They will abruptly rise from the ground after a short delay once you intrude on their nest. Once you hear a thresher rising up, look for it and put some distance between yourself and it quickly.
 * You should only try to attack one from a distance, because their melee attacks are devastating and rarely leave survivors. They have a sizable range beyond their appearance due to powerful infrasound. The ranged acid spit attack is fairly easy to dodge with propulsion jets as it is slow, as well as causing less damage.
 * One strategy to eliminate a thresher maw is circle-strafing and shooting the Mako's cannon at it. If you must stop to snipe, time your stop immediately after a spit has passed you by, or otherwise when the acid has a trajectory that won't hit you or your team. It a good idea to use the Mako's cannon as much as possible in combination with the coaxial machine gun, in order to produce the most damage. However the Mako can be substantially damaged or destroyed in a single direct hit from an acid spit. If you are on foot and your Mako is destroyed by acid spit, do not get back into it -- go to the map screen and return to the Normandy instead.
 * An alternative strategy involves timed jumps and holding still. The best way to use this method is to position the Mako atop a large hill along the edge of the Thresher nest. If you choose the hill wisely it should keep the Thresher Maw from being able to hit you with melee attacks. It will continually spit acid at the mako, so all you need to do is either jump to avoid it, or if room permits, simply drive forward and reverse alternately to move away from the path of the acid with each attack. Keep gunning it and use the cannon until it dies. This seems to be the safest way to dispatch one. It appears that Thresher Maws only retreat underground and resurface if you move too far from them in the Mako or if you move a significant amount in any direction (closer or further away). If you are trying to use the stationary method or if you are caught unawares in a nest, keep moving until you hit the edge of the nest and then bring the Mako around for broadside attacking. Usually the Thresher Maw will surface, retreat, and resurface closer to you even if you are just outside the nest. If not, it may have registered you too far away and all you have to do is lure it back out by driving into the nest area and out again.
 * Also, a very good method, when the thresher rises, you stop. you will notice that it doesn't notice you if it is facing the other way. When this happens shoot a rocket at it, it will go underground, then drive fast (circle movement) and listen when it next comes up, and repeat as long as it is killed.
 * Another method still; Immediately after the Thresher surfaces (ideally for the first time), stop moving. Each time it spits acid at you, simply use the Mako's boosters to jump over the attack. If timed correctly, you should be able to avoid the attack ad infinitum. You may incur a very small amount of splash damage using this technique but the amount is negligible at best. If you continually hop over the acid while shooting (movement caused by aiming will not affect this method) you should be able to defeat any Thresher Maw with ease. It must be noted however that this technique relies heavily on positioning--if you are too close, you may not have enough time to react.
 * Also, keep in mind that if the thresher maw surfaces directly beneath the mako, you will die instantly. Watch where it surfaces and avoid that general area to prevent a sudden and unfortunate end.

On Foot

 * If you kill a thresher while in the Mako, you receive significantly less experience. If you want to get the 'on foot' experience reward but still use the Mako's firepower, get the thresher maw down to as little health as possible using the Mako, then get out and finish it off on foot.
 * As thresher maws do not have armour and special resistances, even your pistol can kill one. Use damage boosting skills as much as possible. Using Warp can reduce the thresher maw's defenses considerably. Even if you can't see the thresher's life meter or bullet impact effects on it, rest assured you're doing damage as long as you're aiming at it. Be certain to stay out of its melee range, as it is closer than it physically appears to be. Don't attempt to determine its melee range, just stay back. Keep enough distance between you to enable you to determine the path the acid spit is taking.
 * While circle strafing, your party members prefer to stop longer than you and don't even try to dodge the incoming attacks, so try to shoot while continuing moving in a linear fashion. They may still run into acid spit aimed for you, and you may take splash effect damage off of them, so find a semi-circular pattern that avoids this.
 * Whether combating a thresher maw in vehicular or infantry combat, you should be aware of your odds of survival. When on foot, the acid's splash effect may kill you, even if it doesn't hit you directly. High level Soldier health regeneration, and/or Medical Interface that provides toxic resistance can mitigate this considerably, as well as use of Immunity. Surviving a direct hit from an acid splash requires a very high amount of health, as it completely ignores shielding.
 * If you are fortunate enough to be a Bastion, try using Stasis to buy yourself some time.

Mass Effect 2
Only one thresher maw appears in Mass Effect 2, and you have to fight it on foot. It surfaces during Grunt's loyalty mission on Tuchanka.

Tactics
It stays outside the playable area, and so melee damage is not a concern. However, its acid spit attack travels much more quickly and is much harder to avoid. The attack will destroy most of the cover in the game, however, the two pillars at the end of the platform will remain intact and will block the attack (as will the platform itself if the thresher maw is attacking from the extreme left or right side), so get behind those and slowly whittle down the thresher maw's armour. The thresher is immune to the effects of Stasis.