wave 1 start with harpoon on left and harpoon on right, and bouncer in middle (that's all the mana you have to build with).  once wave starts, gather mana and swap bouncer for harpoon.  build what you can and focus on maxing tower buffs as soon as possible.  Aura's should cover all 3 areas.  If your aura's are too small, put one set at each of the 3 locations and drop the bouncer in the back and extra harpoon at center.  If you do that, you'll have to watch the archer in the back and keep him healed.  standing at crystal looking out, the left side seems to take the least amount of damage, so if you need to reinforce the back or other sides, you can probably cut an archer (or spider) from that side.  Once built, I play my summoner for each wave and phase shift so he takes no damage (repair with no risk).  This also makes all armor drop instead of armor/weapons, which I've read helps with banking mana.  A full cycle through wave 25 has netted me just over 1b mana.  Shout out to onightpineberries for the base build, and I've added some minor changes.