R Reaper are the easiest.

Take a high health Rex and go on Gen2 to the final mission area. And fly around until you find a Reaper Queen and lure it in the acid pit (under the terminal) keep hovering above her and keep scanning her with HLN-A until she has 5000-10000 health, lure her out of the pit then bite her until she glows and does a roundhouse kick. Get of the rex and repeat it until you're pregnant.