Gears of War 3 Horde Boss (Berserker)

The only way to kill a Berserker is NOT with a Hammer of Dawn.

Berserkers have a ridiculously thick skin that causes most weapons to do little or no damage, but a small amount of fire from a Scorcher causes its skin to become permeable to other weapons. Most of the time in horde, a free Scorcher will spawn during a Berserker boss wave - simply use it for a few seconds on a Berserker until its skin looks molten (black and orange), then use another weapon until its skin returns to normal. Repeat as necessary.

Grenades and other explosives will also damage a Berserker, even when its skin is normal, but they are still more effective after the application of a Scorcher.

same answer as tom, when the boss wave starts whatever you need to win will appear at some point, so someone grab the scorcher and set about wiping out all the other enemies first, leave the beserker alone until theres only 1 remaining. the person with the scorcher pops a few seconds flame down on him and move the remaining 4 players can shoot him in the chest with whatever weapons they fancy,