I use this build to farm giraffes and earn XP for any characters I'm levelling. I leave my apprentice active and swap out 3 characters, which earns around 33 mil xp in 10 mins. It's not quite as fast as getting XP from Summit challenge, but you can AFK once it's built. If you have less than 1800 Fortify on your monk/huntress you might want to repair after wave 24, or during wave 25. Apart from a few tweaks, credit goes to Kingar for most of this.
All gates get:
At each gate, upgrade the Explosive trap 3 times and the Strength Drain 2 times
All DSTs get 3 upgrades, except for the 2 that cover the copter lanes (hint 2), which get 4 upgrades