Episode 1: 1 | Workers 2 | Frog 3 | Mosquito 3 | Guard 4 | Rocket Rat 12 | Stationary Sentry Gun 13 | Orange Hazmat Guy 15 | Laser Drone 18 | Small Tank 18 | Laser & Spike Robot 20 | Missile Robot 25 | Big Red Melee Robot 45 | E1M1 Big Mosquito boss 50 | Turret panel 52 | Mutant Dog?

Notes: Katana kills don't give XP. First four levels are at 500, 1000, 2000, 4000 XP.

Could do the rest of the episodes but only the first three levels seem interesting for the run anyway.