Pastshuurs are best farmed alone. If you have some tanky crit build with medic perk you don't need stimpacs at all. You can get 3000 xp for each if you have swift learner and loner, once you reached the level you need reroll to a real char. XL is good against them, m60 too but the ammo is harder to get. If you also farm enclave patrols you will get shitloads of 5mm ap ammo so you should be fine. Before you go on farming you should do the quest from the doctor in junktown (retrieve scorpions eggs), it will increase your poison resistance. You need simply a combat armor, full drugs (take psycho AND beer), the helmet isn't very important because they will always spit their spikes in your body, they cannot aim a specific bodypart. Play real time ad run for hex, since their body is bigger than a hex, it can be a bit tricky to know if you are at point blank or not, so check your damage: 100-150 you are too far, 300-450 you are good. Having critical hit chance at max will make you gain some time, they have around 1200 hp, it seems scary at start but they don't hit that hard if you have normal DR and they loose their HP fast: you can expect 350 damage on regular hex, 500 to 700 on crit. Thus you will need 3-4 hex to kill one most of the time, sometimes 2. Also, that one is important, don't forget to take a tool or any repair item. You will not loose tons of HP, but you will take ALOT of little hits so your armor will deteriorate really fast, keep it above 55%.
That's all you need to know, good luck.