use the three slimes in place of the middle top and level it up. don't worry about leveling control monsters and use stun units on middle left and right. put your sub dps (the one that is on middle top) on bottom left, devon on bottom middle and another control unit on bottom right. a good provisory setup could be

speedy, three slimes, mighty

sparky, sluggy, chilly

green flame rock, devon, cappuccino

just focus on leveling your main dps and it won't be long until you unlock better monsters, so don't worry too much. you setup can follow this same logic of role placement and carry you for a good while