I started a game with infiltrators as my bonus ship, high ship caps, no enemy waves, blitz attack speed and a 10-planet snake so they'll hit the core worlds really soon. Just looping three space docks (one for each mark of infiltrator) and sending a constant swarm towards the enemy worlds. Should lose 30000 eventually.
This does feel like gaming the system a bit, though, but I don't see any other reasonable way to lose 30000 ships while the achievement is applicable for the promotion.
Lost about 3,000 so far.