I'm getting 40mins combat and 45mins total time with this build for waves 21-35. It's probably only worth farming this during MOTW or for Genies. The stats are my hero stats, probably fine with less.
Positioning the tower stacks to the west of the cores is a couple minutes faster than placing them inbetween the cores. The times can be faster with better builder stats and it may be worth trying some traps.
I have the TB, 2 guardian summoners and a countess builder out during the waves. The countess is afk at #3 facing the wall.
TB often runs out just before the last 100 enemies, if you turn it back on, you can get faster times but it's not necessary.