The Fire Reapers are blind, but have keen hearing. They also cannot leave the lava. Any creature that moves next to the lava will alert them and they will all take a step forwards and attack if they are in range. They are very strong, so fighting them is often not an option. You need to allow one creature to travel through the path in the middle in the lava (choose your creature with the most APs!). To distract the Fire Reapers, use other creatures at the side of the pits. Alternate between your creatures. Move the creature in the center path one tile, and then move your distracting creatures one tile. This keeps the Fire Reapers confused, they will alternate between chasing each creature but won't be able to get close to any of them!
Tried this strategy different ways. Minions or lock in each pool corner, high def moved down middle, reapers seem faster than distaction. Even threw in an elephant. Started looking around for ways to destroy the crates or other hidden levers and was ready to give up. Will go back to attempting the distraction technique. Thanks for the hint, at least I really was on the right path to begin with.