S P O I L E R
Easiest place - last battle with Siren in front of the gate to the Comstock's house.
Get on the balcony above the sniper rifle tear and hide behind the left column.
There is a Mosquito Tear flying above the square. Keep calling it while Siren resurrects enemies:)))
Just watch out for the melee enemies - they sometimes run to your position.
Got it on 1999 difficulty.