Given their preprogrammed response to fight to the death, most aliens will eventually leave their UFO in an attempt to kill X-COM soldiers. This does take time, however, allowing you to set up a UFO Exit Ambush. However, sometimes, aliens will be perfectly content to sit in their UFOs and wait for you to come to them. This is especially true of any alien who is capable of psionic warfare. This page is a guide for sweeping UFOs depending on species and UFO type.
- Aliens cannot come down a gravlift if a soldier or HWP is standing on it. Use this to your advantage.
- Aliens are highly reluctant to fire down a gravlift with any weapon except the Blaster Bomb, and then they may run into the CE Blaster Bomb Vertical Waypoint Bug and often kill themselves.
- Explosives tossed to the top of a lift shaft and allowed to detonate allow you to clear any elevator ambushes they may be preparing.
- Grenades can be tricky to use in a UFO because of the confined space, but feel free to do so around any compartment that isn't housing Elerium. Most UFO components can be easily manufactured, serve no in-game purpose aside from sale, or both.
- The first person through a dangerous door should meet as many of the following conditions as possible: He should be wearing armor, have good reactions, and be expendable. If all else fails, rookies may have to take one for the team.
- It's noted elsewhere on the wiki, but if you fire an explosive weapon round into the ceiling, the explosion takes place on the level above. This can be used to clear bridge-spawned camping aliens in larger UFOs, if you can be bothered to carry the appropriate weapon into battle.
- It shouldn't need to be said, but making your own door into the compartment is generally safer than entering through an existing door.
- The only weapon that can open a hole in a UFO outer hull is the Blaster Launcher (and the High Explosive, if you're playing with XComUtil's improved weapons). It may be worth carrying a few along in the cargo hold just to enter this way.
Specific species have specific cautions when clearing UFOs containing them.
In the tight quarters of a UFO, the Floater's flight ability presents no challenge except when it sneaks around with it in the Terror Ship and Harvester. Aside from that, most Floaters have pretty low stats and are little more than cannon fodder for a UFO exit ambush. If you do have to go in, they aren't a major threat.
Reapers change the nature of a UFO Exit Ambush somewhat. Their massive constitution and health make it very difficult to effectively reaction fire with most weapons. Concentrated fire during your turn is your best bet, possibly recruiting Floaters to assist in the fight. However, as a large unit, the Reaper has a difficult time getting around in a UFO.
Snakemen are slow, have mediocre stats, and have no special abilities. A UFO exit ambush will often work. However, if they aren't coming out, you'll have to go in, which still isn't a huge problem. Given low reactions and TUs, they can be easily ambushed and shot down.
The Chryssalid entirely changes the nature of UFO sweeping. A lone Chryssalid can convert an entire fireteam into Zombies in a single turn. They also have extremely high TUs and reactions, making them an absolute nightmare in the tight quarters of a UFO. If at all possible, try and make sure all Chryssalids are accounted for BEFORE you enter the UFO; else, groups making small movements ending in good reaction fire positions are your best bet.
Mutons present no special hazards when sweeping a UFO. On their own, their stats(aside from health) are fairly average, and they don't have Psi, so they can often be outlasted. As well, they're easily Mind Controlled, and this can assist your troops in sweeping the UFO. Having an alien take point is safer than having a soldier take point. In fact, a Muton's Health score is so high that even if shot while taking point, they can often survive to return fire on the attacker!
The Celatid is not much of a threat either, nor is the Silacoid. These two aliens only present a mediocre increase in difficulty, the Celatid being the more dangerous of the two.
Time is NOT on your side when fighting Ethereals; unless your Psi Team is truly powerful, you will likely have to enter the UFO and sweep it quickly in order to avoid being psi'd to death. Ethereals have fairly high stats, and often have a significant number of Leaders, so you'll want to use verteran troops and HWPs to clear the UFO out.
Sectopods, when present, are an additional challenge, as they are difficult to affect with Psionics, never lose Morale, and are heavily armored and thus difficult to take down. An explosive or shooting it in the back is probably your best bet.
If the alien is inside the UFO, stand underneath him in the gravlift beam, and shoot up the gravlift until you kill him. Simple, easy, and effective. Completely safe, as well, assuming you don't let him come down.
Not much procedure here. Only one room, so enter at the beginning of a turn and spray the occupants with weapons fire. If the power source and Elerium have already been destroyed, feel free to open the door simply to toss in an Alien Grenade or High Explosive and let that be your clearing solution.
Getting a bit trickier, the Large Scout has the infamous 'camping alien' who hangs out in the one small room on the lower left corner. Procedure dictates sending two groups in, one to clear from the top, the other to clear from the bottom, meeting in the bridge before assaulting the UFO Power Source room together. The Engineers will often be armed with Small Launchers, which will allow you to easily capture them by tricking them into firing the weapons at close quarters.
There is only one entrance to the Abductor, so sweeping is fairly staightforward. Enter, sweep the entrance room, then the Examination room. If you have Flying Suits and Heavy Plasma, you may want to enter the bridge from the Examination room. Else, split up to sweep both corridors on the ground floor before ascending to the second floor, sweeping the dead end corridor, and then attacking the bridge with full force. Psionic aliens will often camp in the bridge, so the Abductor makes a prime location to take advantage of the explosive glitch mentioned under "General Strategies."
The Supply ship should be stormed in three phases, one for each floor. First, enter through each door on the ground floor and clear the first floor. Be sure to spend extra time clearing the Power Source chamber. Once you have men stationed on both Gravlifts to prevent aliens from coming down, send a team up to the third floor to wipe out aliens on the bridge; send some men up the main corridor, and one or two down the winding corridor and meet in front of the bridge. A grenade makes a good solution to any guards posted at the bridge door. Enter the bridge and kill any aliens inside. Then clear any remaining aliens on the second floor in a pincer attack from both lifts simultaneously, assuming you have Heavy Plasmas. The reason for this order is that the second floor sometimes spawns no aliens, and has many hiding spots behind tubes, and is thus most dangerous to clear. In addition, psionic aliens usually stay on the bridge, and they are a high priority target to avoid a disatrous failure of the mission.