i will give you a couple tips from my 1999 run-
1. GET WINTER SHIELD, most of the tough battles have sky hooks / lines that you can grapple on, makes them a sinch.
2. snipe lots
3. for the siren battles, don't sit back. kill them as quickly as possible. if they live longer than a minute, you are doing it wrong. i would unload on them with the hail fire (fully upgraded), a hail fire with full ammo should kill them
4. always have at least 1,000 dollars on you to be able to respawn
Seriously 1, 2, & 3 !!!