Boomers are very large and slow enemies in Gears of War. They can absorb a ton of damage, and are always wielding Boomshots, making them extremely dangerous. They are first seen in Act II and also appear in Acts III and IV. Before making an appearance Marcus will usually shout "BOOMERS!!" to warn the squad of the imminent threat.
On any difficulty other than Casual, one shot from these creatures will kill you, so staying in cover is a necessity (you can tell when the creature is about to fire, because it will yell "BOOM!"). The best way to deal with them is to hit them with a Torque arrow or Boomshot rocket. Longshot headshots also work well. It is also possible to use the Hammer of Dawn on them at one point in the game. Unfortunately, when you first encounter these enemies you won't have acess to these weapons, so you will have to make due with Lancer and Shotgun blasts.
Another good method when taking on the Boomers (and possibly the quickest way to defeat them) is to have grenades equipped and tagging one of them after flanking around the side so they cannot react in time. A successful tag with only one grenade will result in a quick death for the Boomer. Be sure to get away from them after initiating the tag because oftentimes the Boomers will instantly turn towards you, ready to fire.
In Gears of War 2, 4 other variants of Boomers make an appearance, one being the Mauler. The Mauler, as it's name implies, mauls you with an explosive flail and is highly dangerous at close range, as the explosion the flail emits can instantly kill you. The Mauler also carries a Boomshield, which it uses to defend itself when shot. Kill the Mauler and you can take it's shield, which is quite useful indeed.
As Maulers rush you when close (and are pretty good at it by the way) you want to keep a distance between them and shoot them from a distance. Use the Lancer or Hammerburst on the legs, or use the Sniper on it's head.
The second variant of Boomer in Gears of War 2. Grinders use the Mulcher Chaingun and can kill you in a matter of seconds on any difficulty level above Normal. Grinders shout:"GRIINNDD!" when they are about to open fire and start shooting, so hear the shout and HIDE.
You want to wait until he stops firing and either repeatedly shower his head with Lancer bullets or shoot with the Longshot. Both are very effective.
Butchers are relatively easy to fight, as they are extremely slow and can only attack if you are right in front of them. However, Butchers do a considerable amount of damage if on Hardcore or Insane, so if you get killed by a Butcher you deserve it, as they shouldn't touch you at all. Butchers are only seen once in the camapign.
Lancer to the head or any other weapon. Anything works on them, even Snub Pistols. Just be sure to evade by diving when they prepare to slash you, as Butchers are never alone and one slice+Hammerbursts=Death.
Flamers are the last new variant of Boomer in Gears of War 2. They wield the Scorcher Flamethrower and thus are deadly at close range. However, their weapon of choice also yields a fatal weakness; the fuel tanks on it's back. Shoot it enough times and it blows up, instantly killing the Boomer. There are also drone versions of Flamers.
Shoot the tank in the back, as that instantly kills the Flamer. You can also take it out as you usually would for other Boomers, but NEVER engage this enemy on Hardcore or Insane up close. They are never alone and thus retreating must be taken with caution. Also keep your distance from them after you shoot their tank, as the following blast can kill you as well.
Related ThreadsAbout multiplayer skins (Boomers) - last post by @ Nov 11, 2008
Boomers + Chainsaw? - last post by @ Jan 2, 2007