I just killed the endboss with a rather simple tactic… My Monsters: Gigarock, Griffclaw, Islanddoe.
With Criffclaw i healed the poison at the beginning and with the healing bomb I got rid of it. With Islanddoe I upgraded the speed of gigarock and got rid of it through stun bomb. From then on it was just “Last stand” Spamming… Took me just a few trys to find out which to kill First and so on!
Sorry for the Not so Perfect English and greetings from Germany !