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.