With permission from runner Hodgey:

The broad strategy is to let the heroes defeat the green keeper, and use the horned reaper to defeat the blue keeper. Straight way I use the corner exploit (whereby digging out a space diagonally adjacent to water or lava leaves a space to move creatures through, which closes up when the walls are fortified) to get an imp outside without letting the heroes in. This took me several attempts, as the heroes can scare away the imps doing the fortifying, and so need to be distracted by suicidal imps...

1. Dig north and use the corner exploit to get an imp into the water, then dig toward the green keeper.
2. Dig around to the high-level heroes and release them.
3. When the hereos are pathing to the green keeper, dig into the blue keeper's dungeon, and then to the horned reaper.
4. Possess the horned reaper and get to the blue keeper's dungeon heart, avoiding the boulder traps (and watching them kill his own creatures).