Pixie + M2875 + a105 is the cheapest combo, and you can *easily* do young / mature.
you need a fap for olds though, and i would also recommend upgrading to rascal armour.
Guardians = you need a long range weapon aswell - something like MKII or FF8K. Then hit em with 2875 when they get close - pretty good way of killing them
Adults - gonna need more armour, say nemesis, + the killing combo from before + a better fap, maybe fap80 + ek2600
Dominant - hehe. Nemesis + ek2600 + good long range gun, say DAR9300, and a very fast carbine / pistol, like imp 2870 or adj v1.
Alpha - angel + whats above..
AOA - Angel + Imp 8k + Imp 2870 + adj fap should be ok
Prowler / Stalker - Shadow + Imp fap + Mod Dar + Vat100ME w/ Dante / Mod Merc + 204 / Mod Tegretov TR6 w/ Dante
But to be honest - just get skillies first