Night Terror
If you're playing Night Terror, you really want to have hit the 450,000 point mark- which means the ability to buy a Corrosive right away, and access to the Circular Saw later on. True, using the Saw means you'll gain almost no points from the round, but you're looking to farm scrap, you're in the wrong mode. Night Terror is about pure survival. The pattern is the same as Nightmare, but enemies are tougher & faster, and scrap accumulates slower. The best strategy is to make the first few rounds the hardest- that way if things do go pear-shaped, at least you haven't lost much time. As the game starts, immediately buy the Knife and a Corrosive, and unlock the nearby Compactor, bringing your total of functional machines to three.
Knife the Shamblers. The room you unlocked seems to be the best ambush point, just hangout by the door and keep swinging.
Whenever the coast is clear, dash to your other two compactors. When the kill all enemies sirens go, you'll face a wave of shamblers follwed by three quadfoots. You want to keep you spending light- if you can knife them, great, but the turret near the key room is also pretty effective for a mere 100 scrap.
The next round is essentially waiting until you have enough scrap to buy the Saw. Once you do, use it on everything. Shamblers go down lickety split, and quadfoots still jump right into the blade. Jack's first iteration is also just grist for the mill- he will occasioanlly push you away, but these manuevers don't do any damage. Keep after him with the blade to put him down for a few hours.
Your next priority is the Magnum and it's upgrades. You'll want it for the final battle with Jack, but it's also good for taking out the vomiting Blobulas (one just before 3am, two just before 4am). Other upgrades depend on preference, but of course the compactor upgrades allow you to get more stuff (in exchange for short term deprivation). Other good buys are the first bottle of Adrenaline (the closest thing this mode has to an insurance policy), and the first Speed upgrade. Save traps for emergencies- particularly slowing down the blobulas or Jack. In particular, you'll want to save a nerve gas trap or two for the final Jack fight.
These have an effect similar to the neuro rounds, and will help keep him dazed long enough to fill his skull with bullets. It's takes about a dozen shots with a fully upgraded Magnum to keep him dead, but softening him up with traps before hand can help.
The circsaw is also helpful, but only use it after he's been gassed- if Jack manages to get you in the scissors, it's instant death. If there are any weapons you haven't bought yet (especially the grenade launcher), now's a good time to buy them for cheap and give him a taste of your whole arsenal. Persevere, and victory will be yours.
Which involves almost no scrap because of the saw, and Clancy's predestined death, but at last you get the Sleepless in Dulvey Achievement.