Salutations! -- I've built every one of the builds published right now, and my stats just didn't cut it for them. This was the closest I was able to do (it's allowed me to farm many 32-35 runs), and it involved pulling in a few more elements and trading one of their original traps. Their actual build (with minion walls) folds for me for some reason, but the bulwark saves lives. So many lives. That and moving where the towers are to the cheeky little crystals. (Reasonably sure when I 'remix' something that it shows where I got it from, but if not this is adapted from susususumannn's build)

Here's a screenshot of everything built: https://imgur.com/a/9fCSOCl

Blue arrows are cloud directions.

Gold arrows are mortar directions.

They recommend: 

4 summoners with 2/2/2 app, huntress, monk guardian pets. (I use goblin cupid and imp on my 4th summoner for healing.)

I'm not that fancy:
I use one summoner with a monk/huntress and a TB monk (if I plan on AFK'ing). My stats are lowish for RL. 5k on the pertinent things for the 2011 toons, and 3-4k on the pertinent things on the new toons (Guardian/Hermit). I was able to AFK 2 (3 if I'm actively on the TB monk) other characters for EXP doing this this way. The only true 'safe spot' is on top of the towers now -- Still take a few hits, but definitely survivable. I put my summoner on the wisp dens and my monk on the mortars. Whatever they did with the update makes that splash damage from the big bads at the end nuke everyone even inside of the reflect. 

Last edit: Be careful where you put the Billies (Ogres). If you put them in line with or just beyond the bulwark, some of the swings they catch will make the crystal catch a few extra strays. 

ALTERNATIVELY >> You can delete one whisp den and instead put down an owls nest in that stack. It'll help the end when you're waiting on the mini bosses to die because the NE Bridge LOS's the towers. Plus, who doesn't want to watch owls fly everywhere. With the extra 2DU you can build a better tower safety stack area with a 4 beam hiding zone. :D