Please see my guide for Act One if you are having Delta achievements issues (it will not pop up after you beat levels or game on Delta, there is a tracking glitch in the game seems to only record your 1st time through for difficulty and will not adjust for higher difficulty on subsequent playthroughs).
Hardest part for me was the last boss. Best approach is take out his 4 guns up close using the large wreckage right in front of him and use mini gun on turrets (so pick one up as well as a Guass Rifle and ammo, this will be all you need).
Once they are down run back to the door (now blocked) that you came out of to get onto the deck and there is more mini-gun ammo there, pick it up and now run to you left around the wrecked fighter and through the fire (you will take damage but not die) and keep moving to your left and back and you will see 2 more wrecked fighters near the edge run to these and over the edge you will see a walkway that you can jump down to, get down there, crouch down and wait.
Psycho and the girl in the VTOL will talk to you, wait till they ask for help then CLOAK and stand up, the aliens will not see you, line up the closest one and let him have full Mini-gun carnage and when he goes down quickly crouch down again and max out you health and energy then rinse and repeat on the next two aliens.
If the girl says use the Tac Gun do not do this till all 3 aliens and down and dead do not worry the shield will stay down till you are ready. Also do not worry about the VTOL, the aliens cannot destroy it. Once one side is down it will repeat the same 3 alien thing again so do the above again. Stay on this walkway till all the aliens are dead then jump up and put the second Tac Gun round into the other side.
Checkpoint here. Now Equip the Gauss rifle and run up to the Alien ship, it will begin to rise up and fly over the ship deck. On the other side of the ship is another walkway lower down, get down there and depending on which side of the alien ship you are use the Gauss to take down the turrets first on your side then start blasting the circular lit up hatch on the underside with the Gauss as well (should take several blasts to do this and open a hole). if small aliens attack you just use the Gauss on them as well. Once the Hatch is open put a TAC round inside and you have taken down the alien ship.
Now jump up and race to the end of the deck for pick-up, hide behind the raised jet blast panel near the back of the deck and equip the mini-gun again will full ammo. Hiding here will delay the small aliens from tracking you, use cloak if you have the energy left. Just stay here and kill any aliens that do find you and wait till the VTOL has gotten into position at back of ship then race to the back and get on board. Congrats and enjoy the Cutscene (double achievement time for 120 points).