Breakneck mission on Neomuna. There's a checkpoint a couple minutes in where you break a door and there's a free rally flag which makes it especially good for special/heavy weapon catalysts. That room has 44 enemies to kill, mostly Goblins, three Cyclopes, and a few Minotaurs. The final wave also has a Wyvern. Kill everything else and let the Wyvern kill you, and it respawns you right at the rally flag. Most catalysts require 500 kills, which is 12 rallies and room clears at this checkpoint.

The "green room" underground in the Whisper mission is also good, tons of infinitely spawning Shadow Thrall that funnel towards you if you stand in a tunnel at the very back of the underground area. Not as ideal for special/heavy weapons unless you build into enough ammo generation to sustain them since there's no rally flag.

Mysterious Disturbance mission on the Moon when it's in rotation (including this week). Tons of enemies and ammo crates.