When a Massive Onslaught is triggered, a wave of xenomorphs will be sent at your squad. There’s no escape from it as the aliens will home in on your squad wherever they go. Your only hope is to hold out and kill every alien that’s thrown at you. Here are a few tactics that can help increase your odds of success:
Sentry Guns: Keep one or two Sentry Guns on hand for Massive Onslaughts and point them at chokepoints the aliens will have to come through to get to you.
Welding doors: If you have a moment to prepare, get your squad into a room and weld the door shut. It will take the aliens a bit to break down the door, and in that time they’ll have bunched up. You can blow away a significant portion of them with a single grenade as soon as they come through the door.
Suppressive Fire: Have at least one Marine use Suppressive Fire in the direction the xenomorphs will come from. This will significantly lower their movement speed, giving your other Marines a chance to take them out before they get too close.
Every shot fired or explosion detonated will draw the attention of the xenomorphs. Since Alien Aggressiveness increases as you engage the aliens, it’s best to avoid unnecessary firefights. Choose your battles carefully. If there’s an alien roaming the hall, deploy a motion sensor elsewhere and trigger it to draw it away. If there’s a barricade blocking your path, try to find an alternate route.
There are quite a few resources spread out around every location, but try not to blow through them too quickly. Sometimes you’ll need to Weld two or three doors shut to take a rest. Other times, your whole team will be ambushed and you’ll need all the Medical Supplies you can get. Keep at least two of each ready for when you desperately need them.
Extraction isn’t Quitting
If your Marines are at maximum Stress, low health, and running low on supplies, it’s time to go. There’s no need to force them to tough it out. Bring them to the ARC and select Extract Squad to bring them back to the Otago.
When you extract your squad, you aren’t giving up on the mission. You can still return to complete the mission with a fresh batch of untraumatized troops. Just keep in mind that every day that passes advances the Infestation level of the planet towards its next level.