This is an AFK DST build for monsterfest that can be done solo without ever getting on a DPS, and does not involve exploiting any mob behavior so hopefully is patch-proof. Once it's built, I like to get on a tower boost monk with a speedy chromatic gemstone and stand in position (1). This gives you range to buff all towers and breaks line of sight to charging dark elf warriors, which results in them usually getting killed by DSTs before they reach you. Even if you have a lot of hero health, charging DEWs can be a problem because the attack is AOE, so it hurts the DSTs.
The specific DST arrangement above is what works for me, but all that really matters is that you use that lower area to build towers because they are inaccessible to mobs unless they go all the way around the bottom, which protects you from DEWs and ogre spit. There are 14 DSTs, 5 Inferno Traps, 3 Thunder Spike Traps, 1 Gas Trap and 1 Spiked Blockade plus a 4-aura stack.
Build Phase Before Wave 1: 4 Auras and spike blockade, then switch to Apprentice to build during first wave.
Wave 1: Build top 3 DSTs then build the 2 DSTs next to the (2) marker. You'll need to gather 10 mana to finish the second one. These 2 DSTs are for flyers and should be placed so they only hit flying lanes. Continue building DSTs as mana permits. Use apprentice C buff with right click attack to increase DST attack speed as needed.
Build Phase Before Wave 2: Switch to Huntress, build 5 Inferno Traps in front of Spiked Blockade, shrinking the radius of each (ctrl + mouse wheel down by default) so they are as close together as possible and will stack damage. Switch to Apprentice to build during next wave.
Wave 2: Keep filling out DSTs and C-buffing as needed. The one DST facing east is meant to try to catch mobs that get bugged and congregate at point (3). IMPORTANT: If mobs get bugged here you must kill them by kiting them to your defenses or killing them yourself, otherwise after some time the challenge will count down and fail. Obviously this can be ignored once this bug is fixed :)
Build Phase Before Wave 3: Switch to Huntress, build 3 Thunder Spike Traps, again shrinking the radius as small as possible and putting all 3 right in front of the Spiked Blockade. There is enough mana to have the build finished by now if you were really fast, but if not just switch back to Apprentice to finish building in the next wave.
Once the build is complete it requires very little maintenance besides checking aura and wall health occasionally. Wave 6 can be tough depending on your stats but saving some mana for instant wall repairs and playing a hero with decent health should be all you need for safety. Note that if you're going to play a blocking squire, try not to stand near your towers or wall as charging DEWs will damage them.