You can get this under 30 sec with some luck and patience. There's no real trick here but here's some tips:

Restart until you spawn underground.
There are 2 spawn points underground: One where you're in a dark green hallway and the other where you're facing a large round object near the blue entrance. The dark green hallway spawn seems to work better than the other.

From here pick up the flamethrower on your way to the circle at the junction and stand on the circle. Look towards the red door to see if it opens and if a snowman comes through.

On the best runs, this door won't even open and you'll never see a single snowman. But if one comes through, let it come down the ramp then try to flame it without getting set on fire yourself. If you get set on fire it's ok as long as you respawn underground. If not then restart.

After this, use that circle at the junction as your base. Keep looking down all three paths to see if a snowman is coming. You may even go down the long green path until you see the ramp. You'll know if it's a good run if you can hear continuous spawns above. Also, never run outside.

To get under 30 sec, it might take a few hours or maybe all day. So far, no one has figured out how that level works. When you're underground something happens that makes all the snowmen above die at once, even if they're not near anyone else.