everything touches the buff beams, and everything should be close enough to the center that Tower boost catches it all. TB never fell off, but HB needed to cycle mid-wave, so like 1300-1400 kills in, cycle HB and you're good.
Party:
1 Hybrid Monk (Tower Boost and Hero Boost stat as high as you can get them), standing on the crystal
2 Hunters with cloud pets (auto firing, both facing North), at hint 5
1 Summoner with cat/huntress pets (cat boosts the DPS and Huntress boosts the towers) right in the middle, on the empowering shrine
Combat time: 33:30 - translates to about 120+ kills per minute more than my traditional build