Seeing as how all of the dossier achievements have all of the secondary objectives highlighted, I'll just give some tips on the hard parts (or really, the only exceedingly difficult part of the game).
It's fairly straightforward, and not extremely difficult until the last part of the cradle level (last level) where you have to get by three waves of enemies all while fighting Trevelyan. First of all, unless you're extremely good at the game you're not going to have much health going into the fight. I only had 3 bars of health going into it, but luckily there is body armor right before the fight along with a shotgun and the 3-round burst rifle. I highly suggest taking both of these weapons and the armor before going into the room, because you can't come back into this small room once you're in there.
Basically what happened to me was that I kept getting destroyed while trying to get this done, so I found a couple good strategies (one of them being much better than the other, but is more difficult to pull off. You'll see why when I go into it). The first method (which is easier for the first two waves, but sucks for the third due to the helicopter) is to simply go in, take a left out onto the catwalk, and go all the way around to the far side. This allows you to pick off enemies more easily than running around getting shot. They won't come towards you enough to get a good shot on you unless you expose yourself too much, hence why it's easier to pick them off and avoid damage. Again, though, there is a drawback to this method. During the 3rd wave, a helicopter will spawn outside and won't really come near you but can still shoot you. You then have to haul ass to the elevator and avoid being wrecked by the helicopter. A video of this method is posted in V0CALICMAX's solution.
Now, the easier method that only took me a few tries and allowed me to finish the level much more quickly than above said method. Again, grab the same guns and armor. This time though, go down the steps to start off and run around shooting people (after they start spawning, of course). Once a guy spawns in the bottom right door (right being once you enter the room, slightly to the right and in front of you on the bottom floor) stand and wait for him to come out, melee him, and take cover in the little spawn closet. No enemies will spawn out of this door any more, only the one across the room. If the door to the catwalk on the right hand side of this same closet is open, simply step slightly outside the door leading directly to the inside big room and move back. It should close by doing so. Now just camp in the back of the little closet (crouching in the back left corner will close the door in front of you, which you want in order to avoid bullets) and enemies will come to you one by one and open the door, making them easy to pick off while taking almost no damage. This is a good way to get more ammo, because you will run out of rifle ammo when only going after Trevelyan. Use the shotgun to take them out quickly, and then use the rifle when it's only Trevelyan. This avoids fire from the helicopter too, as it can't ever see you (granted you closed the spawn door to the catwalk).
I finished with almost all 3 of my health bars left (because of the armor you need to pick up before) and made it soooooo much easier to finish the level. I would make a video, but I don't have a capture card or video camera. It's fairly explanatory once you try it out a couple times to see what I mean.
Good luck, I know I needed it.