OK I got it going again, although I am not sure how long it will go unattended. I might have to tweak it a bit... Pics are shown below; here are some tips if you want to set it up yourself:
-- play through to level 20 using your favorite setup. Make sure you have one or two orange towers on the lower left as shown in the first pic.
-- try to get your get your rock elemental killed a few times
before the selloff (you want to have some swamp things on the board at the time of the selloff as in the first pic)
-- right after the tree boss is killed, sell off most of your towers, one every few seconds. You should be selling off your last cannon right after the last of the small trees and orange demons is killed. (it's actually safer to sell off too much than too little; you will have plenty of cash by this time, so you can build a temporary big cannon if you need to clean up some enemies)
-- move your hero and your paladins/barbarians out the way
-- move your rock elemental back just enough so that the swamp thing is
throwing swamp goo at it. Do not move the rock forward to engage in hand-to-hand combat.
-- keep or build one or two offensive towers as in the fourth pic. You want the swamp thing to be at around one-third or one-half power when your rock is killed.
Right before the end of level 20. The lower left part is the most important. The rest of the setup really doesn't matter; you can use your favorite setup. There are actually two rock elementals that have been created here, but they are both dead/regenerating at the time of the snapshot:
Right after the selloff. The towers at the top, middle, and lower right are out of range and are not needed. I had to build a cannon right after this because the balance is a bit too far in favor of the enemies at this point:
Swamp thing throwing swamp goo at the rock. The swamp thing should have a bit more life than I had here; this is cutting it way too close:
A somewhat-stable setup. The paladins are probably not needed. I might need to add a second tower to get the balance just right: