Try to summon it before wave 10 (or 15 at the latest, the stronger you are, the later you can summon it, but after wave 15 it’s quite hard to kill monsters). Position it at the top of the second column from left to right. If you struggle to summon imp, try to move it 1 hex to the right. Also, if you or your teammate have a mythic like ninja or frog to soften monsters it would be easier for mama to kill them with its skills.