Quick, easy strategy. First units needed: archer, kangaroo & bear; when you unlock the Rhino, use/upgrade only Rhino and Archer. When you get the last two units (penguin and dragon) you can use them as well, just keep on sending rhinos and archers in order not to waste much food for penguins and dragons.
Endless swirl, got over level 25 with this and in this order:
SOULSEEKER (40ela,15atk,10atk), SHADOWDRAGON (15atk,15atk), SHADOWDRAGON (30atk,15atk), ENTROPYALIEN (15spd), DEVOURER (500hp,350hp,200hp). Good luck! This eventually works ;)
 "Kingdom Rush Frontiers"){kind=icon}
 "Hack Slash Crawl"){kind=icon}
 "Paladog"){kind=icon}
 "Rogue Soul 2"){kind=icon}
 "Pocket Creature PVP"){kind=icon}