The objective of this Red Faction: Armageddon achievement/trophy is to defeat the mantis.   To that end this guide should help. So here you go:

This achievement needs skills, eyes and hands coordination on targeting planted bombs by using launch grenades. You must consistently hit the planted bomb to blast on and destroy structures. Hitting it consistently will delay the chasing mechanism and attacking of the alien. Explosion of the detonated bomb creates massive destruction of structures and debris, which creates blockage of the rail track and delays the chasing mechanism of the alien.

TIPS –you must consistently blowup the detonated bomb so you won’t be chase over by the alien.

Successive misses on detonated bombs means the more chances you’ll be attacked by the mantis and then it’s gameover. And it would take a lot more time achieving this trophy.

So Stay Focused, be dedicated, and determined to surpass the achievement.

Use LT (hold)   to snap your cross-hair to the nearest target.

As you go further in the game, you’ll receive instructions on the escape from the dig site.

Approximately after 30 seconds you’ll reach the checkpoint.

At this moment you’re at an open door elevator, going upward. Be attentive to the surroundings, there are plenty of enemies around. Shoot and kill the enemy you see right away and turn around every time because there are lots of mobs everywhere while at the open elevator.

Tap LB to launch a repair grenade to repair the generators.

After repairing the generators (the door will open) immediately stay out of the elevators, more attacks will start. At this point, you’re nearly on top of the dig site.

The mantis will show up for a second time. Same strategy will be use (consistently blowup the detonate bomb), mobs are everywhere. Next, destroy the hanging generator so that the rail train will start again.

At the end, Hale will come out using a mantis. All you need is to crash out all power stations on Hale, crashing all of those give huge damage to Hale.

After damaging his by HALE use rail driver to finish him off.

Follow every steps of the guide and your trophy/achievement will be achieve easily. Enjoy playing this intense beautiful perfect game (RED FACTION ARMAGEDDON).