In Arrival of Beasts the bossbeasts are the most dangerous enemy for you. To get more information about them you can look through the bossbeasts listed up in the following enumeration of the bossbeasts:
Much more bossbeasts coming soon ...
The bossbeasts always get spawned by the spawner buildings and mainly try to kill your protectbase. Furthermore the ranged bossbeasts will try to destroy your towers on the way to the protectbase, if the range allows it. You can always click on a bossbeast (in-game) to see all details about it.
Some bossbeasts will be resistant to some module effects.
Bossbeasts are resistant to some of the moduleffects, read more about it on the modul page. Try to kill them with your towers and / or mages. To fight effective against the bossbeasts it´s important to know their attributes and properties.