even though the frog are cold immune....u can still use a cold mage to dmg them....first they are only 25 hp.....lv 5 cold mage destructor will do the job.....
start off with 2 mage each on 1st ground at the bottom mid slot. once u initiate, start to put another 2 mage facing the 2 mage placed (the 2nd ground). when the top mage lv....upgrade them to ice slow heavy dmg. however for now do not upgrade the bottom mage.
hopefully before wave 3, the ice mages are dstructor. or else they can do no dmg to the frogs.....