Here's a possible device;
Minecarts sweep enemies into the holes, bottom level filled with magma (7/7 to start, 5/7 running, 6/7 returned to idle), the smoke won't make it all the way back up the holes (maybe an occasional tile), no power, stacks nicely, enemy-pathed tiles all have linear-accelerator ramps so no jams.
The grates for stopping the minecarts and their control mechanisms have to be magma-proof as the minecarts dump their loads of magma each time they fall down the holes - maybe bonus.
Z=0
Z-1
z-2
x-3
z-4
z-5
z-6
z-7
Might be able to make it smaller depending on the amount/ affect of the smoke that might make it back up the hole. Silver minecarts don't seem to spend enough time in the magma pit to melt - so far.
edit; added; then corrected:
z-1;
minecarts come up from the south onto accelerators shoot to hole ends and fall in and straight down to z-7
z-7
minecarts land on impulse ramp in magma and are boosted up EW straight track ramps to side
z-6, z-5, z-4
minecarts come up onto impulse ramps from the side and are boosted up EW straight track ramps continuing to side
z-3
minecarts come up onto impulse ramps from the side and are boosted up SE (left side) or SW (right side) corner track ramps heading south and up a level
z-2
minecarts come up onto impulse ramps from the north and are boosted up NE (left) or NW (right) corner track ramps heading north and up a level