use the chest on stage 3. beat it in 10 runs.
keep in mind I rerolled for a starting speed discount, so I don't know long it may take otherwise or if it's possible in 15 runs.
but yeah, get 3 units and the chest's speed to 6 asap, pester the eyes for xp and replace a chest once it falls.
once you have that try to get a 4th unit and level the other chest stats until hp hits 5, then focus damage to 7, it's enough for the chests to rack up enough damage to clear the eyes in 1 wave.
I should mention that you should time your spawns so that your units avoid the projectiles. to safely avoid the void, wait for its epicenter to reach the outside of the outer walls of the lane.
dump into units and wait for the board to clear of projectiles and send a chest in each lane to cleanup.