Gears of War 2 Guide - Act 1: Tip of the Spear
Chapter 4: The Big Push
Once you gain control over Marcus, get on the mounted turret and fire at the Locust emerging from the large hole to rack up some kills. Don't waste any time firing at the Brumak because he can't harm you.
A hijacked assault derrick will soon pull up alongside your vehicle. Fire at the Drones onboard and try your best to keep the turret on their end clear. When the two assault derricks move apart, a few grapplers will climb aboard yours. Your allies will likely be able to take care of them, but blast any that you can reach with the Chain Gun. When the bulletproof glass protecting the driver of the hijacked vehicle cracks, direct your fire at the driver until you manage to knock him out.
Get back on the Chain Gun immediately after the victory cutscene. Ignore the Brumaks alongside you for now. A Corpser pops up along the way, but it can't harm you. Shortly after the Corpser encounter, a Brumak will appear in your path; you must quickly kill it before it can gib you with its missiles. Focus your fire on the missile battery on the Brumak's back. When it explodes it will take the Brumak with it. Remember to cool down the Chain Gun when it overheats by holding RB. On the Hardcore difficulty setting Marcus can only take two missile blasts from the Brumak before he's blown to pieces, so keep your aim steady and cool down the Chain Gun immediately when it overheats.
You'll complete the chapter and unlock an achievement once the Brumak is dead.