If possible, make sure to end the combat wave with full mana for the next builders. Otherwise immediately start the 2nd Combat Wave and complete upgrading what you can, but this time definitely make sure to end the combat wave with full mana for the next builders.
Build Phase Starting with Full Mana:
Switch to Summoner and in the overhead view, build the Ogre and Mage Minion to the southwest.
The Ogre stops any stray Harbingers during the boss fight from getting close to your characters. If the Ogre or your characters are positioned incorrectly, then the Harbinger may pass the Ogre and kill off your characters.
Switch to your Warden and start the wave.
Combat Phase with Warden:
Build all the remaining towers, which are just the Warden towers.
Switch to your Adept Upper after this phase.
Following Combat Phases with just the Adept Upper out: