Found a really easy way to do this. Mission 9 Hope in Hadleys you will fight the crusher. After you rescue the marines and hold off the ambush in cargo bay you will go back outside. Once outside you will go through a metal gate, once the gate closes you will meet the crusher. Look immediately to your left and you will see an incline in the corner you can jump upto. Jump up here and the crusher will not attack or even move, so shoot it from there. Your teammates will also be shooting at it, it takes a lot of bullets of kill. It wasn't dying after I nearly exhausted my ammo so jumped down from my spot and it charged towards me. Luckily it died with one more hit.
Below is a video showing you how to do it, credit for the video goes to azGRIMES