Red Faction: Armageddon

Survival Of The Fittest: The boss at the end of this level is the Behemoth. The best combination of weapons to kill it would be the Plasma Beam and the Assault Rifle.
Just keep firing at its head, and be sure to roll out of its way when it begins to charge or throws out the green gas.
Once it is dead, you have to follow the path for a short time. After a while, you will be prompted to hold X to meet Kara. Once you have done this, the achievement will unlock.