The Few, the Proud, the Gob Squad

This achievement is actually not too hard to grab.
There are 3 parts where you will have to defend the Gob Squad.
The first is 2 groups of 3 mobs which need to be aggroed right after the first door is blown up. After you kill those 6 mobs the first miniboss will pop up. Aggro him as soon as possible to avoid any possible attacks on the gob squad. The next part has you collecting various parts for a new explosive for the next door. When you try to loot each of the items, various monsters will appear and need to be aggroed again in order to avoid them going towads the gob squad. (the mobs only spawn once per item)
The last part has various mobs on the top of the cliff and they jump down from the cliffs closest to the second door, the furthest from the door, then the middle one, then a boss from the first cliff. In order to do this part, you need to aggro the mobs ASAP with a large AoE spell, because if left unaggroed they sprint to the gob squad. Once you get to the last miniboss, you're pretty much golden unless you wipe.

Notes:
-AoE ground affects do not harm the gob squad, only direct melee attacks
-You can check to see if you failed by checking all the gob squad's members health, which is 393.4k. Anything less than that means you have failed the achievement.

TL;DR - Just aggro every mob as soon as they appear in order to avoid them aggroing the gob squad. Overall quite an easy achievement unless your group lacks solid AoE and gear.

Reference to "The Few, The Proud. The Marines."
Err!

My group and I thought we had this, but we failed on the very first moment it could fail. Apparently if you don't aggro [npc=71244] they auto-shadowstep the gob squad, therefore the gob squad takes damage and you failed. To succeed you have to have aggro before the NPC's performed their shadowstep.