Mud Frogs are the nasty big brothers of the Hedge Frogs you've already seen. They appear in mixed groups, and if you kill the Hedge Frogs, the Mud Frogs will just summon more of them. For that reason, take out the Mud Frogs first. You could use tier-2 spells like Fira and Aerora to take out the Hedge Frogs while damaging the Mud Frog, but it's questionable whether it's worth the effort.