Use Elemental Ammunition
The ammunition your marines load into their weapons in Aliens: Fireteam Elite is lead, rockets, and proximity grenades. You can also load them with a few rounds of elemental ammunition, which can either be found or by purchasing the right consumables. The most effective elemental ammunition against Xenomorphs is fire, which burns through their defenses and deals damage over time. Against Synths, it is shock rounds as they overload their circuits, leaving them open to a counter-attack. It is important to remember that when you activate this special ability from your inventory wheel, you will only be able to load one of the two weapons that you are currently using, which means you need to think carefully which firearm you want to use.