Map 1[edit]
You will see Rykov fleeing with your mask and locking the doors behind. For a person who wants to take personal revenge he is surely acting strange allowing aliens to do the entire job for him. Never mind, you will deal with both aliens and Rykov. But aliens first.
Jump down and follow a corridor with a "caution" sign. There is a hose on the floor, hard to miss. Follow the hose to the crack in the wall until you come to a T-junction. Both ways lead to the same room, so there is no difference which path to take. Before heading in better save your game and prepare some long-range weapon. As soon as you come out of the hole, a fecehugger will attack you and if you're not careful enough, you're dead.
Sounds weird, but the only thing you must do here is to take a pistol and head out. Be careful! Once you appear from the crack in the wall, another facehugger will attack you. If you don't have a pistol, it won't appear. Jump to the crate right ahead and go to the vent where the facehugger came from. In other words, you need this facehugger to appear to progress through the game.
Follow the vent and destroy the next grille. You're in an airlock of some sort. Look for a control panel, open it and hack the circuit. This will open double doors. Another cryptic part. There doors lead to the beginning of the level, but the panel also opens the doors which Rykov used to escape when you saw him. They are not far away, easy to find.
While you will be walking down the tunnel you will be attacked my some aliens. Now it's time to try out the pistol you've recently got. In the next room you will be attacked by more aliens. Using the pistol there will be a bit harder since they attack from different sides. Try pistol's alt-fire to stun a big group of enemies.
In the room look for some dashboards to find a lever. Pull it to raise the drilling platform and go to the next corridor.
As you approach the next room, a few explosions will occur. Now aliens will fight synthets and turrets. Aliens will lose very quickly, so you can try to kill a few foes using the skirmish to your advantage. Just don't use cloak, it will be useless against those enemies. Optimal strategy here is to destroy two synthets above, then two turrets right below them.
See a huge pipe that leads somewhere between a few crates below? Follow it and watch out for aliens above you. When you go through a tunnel you will hear Rykov’s taunt meaning you're on the right way. Right after his line aliens will appear.
On your way you will encounter a praetorian. This specie was a boss two times already but now you have a pistol to stun it, so it may be easier. Just don't forget that the only weapon that can hurt a praetorian is a combistick. When you have beaten the alien or outran it, follow the tunnel to end the level.
Map 2[edit]
You're in a room with a pool of deadly goo and many platforms above to jump on. There are no enemies here for now, but save often. To get further you need to make leaps from platform to platform by jumping while crouching. If you don't know where to jump or cannot leap high enough, use your night vision mode (the last one) and search the whole platform, sometimes the needed spot is near a wall.
At some point you will jump on a broken platform which is near the ceiling. Go to where it's broken and jump on the platform right below it. From there continue your ascent. Pretty soon you will have to jump down once again to continue going up. At the very top you will have to jump to a lower platform again. From there go into a cave. Surprisingly, it's impossible to see the cave from the previous platform using a night vision mode because of its short range. Turn off the night vision and you will spot smoke at a distance. That's where you need to go. If you don't see the smoke, you're on the wrong platform.
Go through the smoke to the cave. Rykov will warn you about aliens. He will also kill a few before fleeing. What a nice guy! Go ahead killing aliens and you will come to a nest where a few eggs are stored. You will find a few of such on you way. Remember that eggs are not highlighted white while you're in alien vision!
Note: Be careful when standing next to entrance to an alien nest. After a few seconds a facehugger may spawn right behind you most likely meaning a certain death. It doesn't seem that there is a pattern in its appearance. Sometimes it spawns, sometimes not.
Exit the nest through the opposite cave and continue until you see an junction. You can go straight ahead or you can turn left. Both ways will get you to the same nest sooner or later, the left way is just one nest shorter. If you hear Rykov shooting his minigun, you're on the right track.
Soon you will exit alien nests and find yourself at a chasm. Cross it and leap to the middle huge urinal-looking corridor. Cross the room which looks like an alien nest but without aliens. Follow the tunnel until you arrive at a tiny underground lake. From there leap from ledge to ledge until you reach a huge corridor. Follow it and drop down.
Boss fight: Huge Praetorian
It's just like a regular praetorian but bigger. It has more health and its attacks deal more damage. The only weapon that can deal damage to this alien is still your combistick. However, now you have pistol. Use its alt-attack to stun this foe and strike it a few times until it recovers. Other than that, the tactics is the same.
When you deal with a huge praetorian, four aliens will attack you. You won't have time to recharge your energy or heal yourself. After the boss fight dealing with a pack of aliens might be as hard as killing the boss.
Follow the tunnel opened by aliens and...
Boss fight: General Rykov
To begin with, he can see through your camouflage, so don't use it. He is also not highlighted when you use your human vision. However, he can be clearly seen when you use night vision. Rykov is the only enemy that cannot be stunned, so don't even try. As for the fight, it's not hard dealing with Rykov due to a lot of pillars to hide behind. Use all the range weapons you've got. When you're low on health or energy or just need cover, hide and restore.