for cave 5, the goblin cant be killed. so you have to get alot of units to knock it back. use golblins since it spawn fast and can tank the hits from the 1hit goblin.
wow i cant believe ppl dont like this game just cuz they dont know how 2 kill darks
IF you want 2 kill darks:
use something besides low leveled pellet towers-like bash, lvl 3+squirts