The Blue guys, with the cricket bat, the orange bandana AND a powerful shield(?!) seem extremely hard for only chapter 4. They resist almost everything way too easily.
Solution: Get Nick and other fantastic damage heroes.
The shield grants the creep immunity to physical damage. It’s important you place heroes in your team that have access to fantastic-based damage attacks to counter it.
Later on in the game you’ll encounter skeletal warriors who have the reverse; a shield that protects against fantastic damage instead.