Using Rion's Spore Bomb, group at least 3 units together in its blast zone. 2 enemies and one of you're units can work, but there's an easy way to get 3 enemies together.

First: Open Death's Door right next to the red entrance to the left.
After that, use Death's Floor to teleport any enemy to the door (I choose the other heavy turret)


Second: Use Transference on any target in the top left (one of the turret's or the shotgunner) and then target another enemy (or ally if you really want) to take the hit.


Third: Throw the Spore Bomb so that 3 or more units get hit by it's blast


Fourth: Beat the level. Usually I open up with a spectral skull to kill the other enemies on the level before I do anything. Use Jen's broom and mobility to lock up the right door, and you can use Rion's Rabid Bite to rush to the left door for a quick clear (also completing Jen's Optional objective). Clean up the other enemies and you should be able to clear within just 1 turn.

Hope this helped :​)
(Also, if the turret's are bugged, then I assume this guide will be out of date very soon)