you do the level after you start walking into the corridor 2 raptors start coming after you. after you kill them a portal pops up.
wait till the raptor pops up then kill it.
to the right of the portal is an opening were there is a dead tau body with grenades, health and rocket launchers. this is one part of the secret task.
continue along do not go to the right!!!!!!!! go straight until the screen starts flashing
when this happens turn around and be ready for a chaos marine with a rocket to come after you.
best way to kill chaos marine is to hit them with a rail rifle to the head.
another 3 marines with bolter's will pop up kill them but move constantly. move onto the top of the platform where the 2nd portal appeared and there is a ledge jump onto it and you have found the second tau body with more health and ammo.
continue along and a chaos sorcerer will appear. just hit it 3 times with a rail rifle for it doesn't matter where you hit them it will still take 3 hits to kill.
there is another platform where the sorcerer was. jump onto it you have completed the secret task.

continue and a another sorcerer will appear. kill it.
go down the corridor you will find a large room, it contains: red medikits, rail rifles, chaos bolters, autoguns, meltaguns, rocket launchers and more bolters.

when you attempt to exit the room, 2 chaos portals will open and 2 sorcerers will appear. kill them. as soon as you kill them, three raptors will drop down. kill them too. afterwards, a chaos dreadnought will appear, destroy it. as soon as you have done that, pick up the meltaguns and kill the obliterator which will appear. CHECKPOINT

after all that, exit the room and move towards the red medikit. DO NOT PICK IT UP UNLESS YOU NEED IT! a chaos portal will appear and seal the corridor. two raptors will drop down. kill them. move towards where the red medikit is. another chaos portal will open up another room.

the next room has loads of bridges, each corridor has a chaos marine with rocket launchers. kill them. when you reach the bottom, there is an elevator. go down and 2 raptors will appear, kill them. as soon as you exit the elevator, another 2 raptors will appear form behind. you know the drill, kill them.

head towards the other side of the room and a chaos portal will appear, move back to the other side as 2 marines will appear, and a dreadnought will appear as well. drill time. CHECKPOINT

head down the elevator and follow the corridor until you find admiral Constintine. he will talk to you about what is going to happen. he'll turn into a chaos spawn, you will have to kill it as well as the load of raptors and sorcerers that will also come at you.
when you kill it, go down the elevator on the other side.

when you arrive down, head to the right and go down a flight of stairs and you will finish the level.

TO GET RANK "A"
Time: less than 15:00
Kills: 36
Accuracy: 60%
Additional task: complete