1999 mode isn't that hard in most cases, but there are a few areas where help is necessary. I would highly recommend NOT using the dollar machines and getting the missable acheivement at the same time.
Troubled AreasIn the vault.An easy method, that worked for me is to get rid of the bodies from Mrs. Comstock so she cannot keep reviving them. I did this by walking to the vault door, shooting at her to activate from INSIDE the vault and then running BACK through the back entrance in the wall. I then circled around and went up the stairs and entered the boss room from the patio. Again i shot at her jumped off and went back in the hall. If you enter from here, you can leave the room which offers shelter. and all the enemies will follow you up the stairs, and if you kill them here, she will not follow and therefore once they are dead, she will not revive them, and then you can easily kill her.
Fight outside Comstock houseMUST HAVE UPGRADED Devils Kiss FIRE TRAPS. Lay 5-10 traps to the left of the statue as shown in the video. As soon as she spawns, she will be killed. HOwever, these must be upgraded for maximum damage, and an instant death. (see 4:41 on video of actual placement)
Command Deck - made easyThis is VERY EASY! Make sure you have the upgraded RETURN TO SENDER trap. Then immediately upon starting, encircle the blue glowing machine with traps. As long as an enemy doesn't get too close, they will take 95% of all damage for the entire fight. And then possess everything that comes close and prioritize your bird to attack anything with a rocket. It helps a lot to have the clothing perk that gives you salt for your kills so that you can keep up the possessions indefinately.