Dretch head shots do 96 Damage. Whatever you are, aim for the head.
Basi grab is a bitch, its a lot easier in 1.2, but sadly no one really uses it in 1.1 much. You just have to look at there leg while touching them. Do it from behind or you die. You need to look up so as to hit them in the torso not the legs, but if you look too far then you lose the grab, so a bit of practice is needed.
Poison deals damage over time to them, depending on there armour. If they use their med kit it cancels the poison and gives 30(?) seconds immunity. Always use poision if you can, aside from anything else it makes a dretch head shot deadly to non-helmeted people.
Hiding and sneaking is the best way

. People never look up, so sit stick to the walls and ceiling as much as you can.
Don't forget they get radar at s2 though

.
Also, poison skirmish works well, run by and poison them, they instantly blow there medkit, wait a bit then re poison.