Boomers have rocket launchers that usually kill you in one hit. In order to kill them safely, take cover and wait for them to shoot first before you start shooting back. Just as they start reloading is the best time to shoot them since their gun has a very slow reload. After they reload, hide again and let them fire. Repeat this a few times to kill them.
Take cover and wait for him to reload, then use a sniper to shoot him in the head. It will explode after about three shots.
Take cover and wait for the Boomer to walk up to you, then let him fire so that he can reload. Once this happens, equip a grenade, run up to him, and punch him with it (and therefore stick it to him). Dive and roll out of the way.
With a Toque Bow, hold the trigger until you see the arc straighten. This indicates that the bolt will stick. Then, fire it onto the Boomer. It should kill them in one hit, even in higher difficulties