Hi guys.
I was struggling with this achievement for a loooong time. I tried different approaches and it finally worked. For everyone struggling as well here is some kind of a guide...aka I did it that way.

My formation (current event: littlefoot freeplay)
https://i.postimg.cc/2jB8yhrW/Untitled-1.jpg
There is one spot left above the Bushwacker, so Mindy just mimes Skeletom

The Bushwacker:
Level as high as possible; All skills

Mindy the Mime:
Level 300; Skills: Mimic; Perfect Mime

The Game Goblin:
Level 50; Skills: Cave-Bound; Bad Games

Brogon; Prince of Dragons (Healer; Bruno/Healerbuffer):
Level 75; Skills: Cleansing Fire; Spark of Live

Dark Gryphon (Tankbuffer):
Level 50; Skills: Under my Wing

Fire Pheonix (Brogon/Healerbuffer):
Level 150; Skills: Heart of the Phoenix

Thalia, the Thunder King (Tank):
Level 75; Skills: Lightning Rod; Godly Healing

Skeletom:
Level: 400; Skills: The Skeleton Reaper; Exploding Skeletons; Quick Bones

Bruno the Unicorn (Healer):
Level 600; Skills: Let my Horn heal you; Dragons sure are friendly

The Holy Guardian (Healer; Tankbuffer):
Level 100; Skills: Undying Protection; Holy Light

Rosie the Rocklechaun (Tank):
Level 800; Skills: Stone Form; Lingering Stone; Porous Stone

Xander, the Xmas Dragon (Bruno/Healerbuffer):
Level 10; Skills: none

And then clicked away all none skeleton monsters as good as I can. And then boom.
In the new world for like an hour. Hope that helps someone.

Have a nice and sunny day