Ok. So I'm past the nuke attack.
If I head outside, the exosuit is already there and Ive got to fight it.
Once it's down, the alien vessel appears. I shoot down its four turrets and then Helena tells me to wax that thing with the TAC cannon....
Problem is I don't have it and from the youtube videos I've seen, You get it from getting down the elevator into the hangar bay.
This doesn't happen on the 360....

Im stuck...
So i turn to MyCrisis community

Nevermind.

The last level was glitched.
When I selected Restart Level, I would still glitch (exosuit already on the deck when I first go outside, so I had no option to go to the hangar bay and lower decks)

I had to go back to the main screen, select campaign and choose Reckoning from the level list to fix this bug.

I spent 2-3 hours on that bug (I Think I killed the exosuit and the four turrets of the vessel at least 7 times before I found a way to fix that glitch.)

P.S : .... Glitched achievements.... I went through the whole game without crosshairs, with the enemies speaking Korean thinking I was in Delta Difficulty (My game settings even showed Delta difficulty) but I realized, when I went through the level select, that I was credited with Hard difficulty only.....

Same boat as you, though no matter what I do the door leading down is shut over.

The admiral tells me to go to the flight deck, but doesn't follow just stands there, then when I smash the he'll out of the closed door leqading down to the tac gun, he appears & gets killed o.O

Tried restarting & starting from the level select to no avail :@ I'm playing on hard.

Ok don't punch the door lol & just go outside, or the level glitches & you can't get the TAC gun.