A good area I found to get this was at the last fight with Lady Comstocks ghost just before you enter Comstock House. There is a mosquito ally in the sky you can summon there and lady comstocks ghost will keep summoning more enemies so you shouldn't run out. I just sat at a good distance from the enemies behind cover with a carbine to weaken enemies so the mosquito could finish them off. If you run out of carbine ammo, Elizabeth will eventually give you more ammo. Note you can summon allies through a tear an infinite amount of times so just keep doing this til you get the achievement.