I explain one of the most effective ways to take out large groups of enemies as an Infiltrator whilst receiving little damage.
Okay, first things first, you're gonna need to have your cloak as high as possible, the longer it lasts the quicker you can manoeuvre around between cover.
Start by hiding behind cover and sending your squad mates to either the far left or right of your enemy in order to distract them. It's key that you stay out of sight and only leave cover when you are cloaked in order to be hidden from the enemy group, continue to cloak and move from cover to cover until you have fully flanked the enemies in the direction opposite to where your team are.
Once you are in position be sure to equip your sniper and any effective ammo buffs you might have, now, target the enemy furthest away from the group with the lowest health as these will go down quicker and be less likely to draw attention to you, headshots are a must.
As soon as you shoot the bullet drop back behind your cover and let the cloak ability cool down. As soon as it returns you can take the next shot. Repeating this method, systematically destroy every last one of the enemies, if an ally goes down be sure to use some meds and revive them as you need them for a distraction.
If the enemies focus on you too much, make your squad use their powers to damage the ones closest to you, that should draw them away long enough for you to cloak and run to another piece of cover. Hope you find this useful guys.