A 5-mana 6/6 flier is fine, though a step down from Doom Whisperer and the like
Use them to shore up your deck's weaknesses
A: You lose the game immediately

[1] Bug Weak against : Fire, Flying, Rock Strong against : Grass, Dark, Psychic Dark Weak against : Bug, Fairy, Fighting Strong against : Ghost, Psychic Dragon Weak against : Dragon, Fairy, Ice Strong against : Dragon Electric Weak against : Ground Strong against : Flying, Water Fairy Weak against : Poison, Steel Strong against : Fighting, Dark, Dragon Fighting Weak against : Fairy, Flying, Psychic Strong against : Dark, Ice, Normal, Rock, Steel Fire Weak against : Ground, Rock, Water Strong against : Bug, Grass, Ice, Steel Flying Weak against : Electric, Ice, Rock Strong against : Bug, Fighting, Grass Ghost Weak against : Dark, Ghost Strong against : Ghost, Psychic Grass Weak against : Bug, Fire, Flying, Ice, Poison Strong against : Ground, Rock, Water Ground Weak against : Grass, Ice, Water Strong against : Electric, Fire, Poison, Rock, Steel Ice Weak against : Fighting, Fire, Rock, Steel Strong against : Dragon, Flying, Grass, Ground Normal Weak against : Fighting Strong against : None Poison Weak against : Ground, Psychic Strong against : Fairy, Grass Psychic Weak against : Bug, Dark, Ghost Strong against : Fighting, Poison Rock Weak against : Fighting, Grass, Ground, Steel, Water Strong against : Bug, Fire, Flying, Ice Steel Weak against : Fighting, Fire, Ground Strong against : Fairy, Ice, Rock Water Weak against : Electric, Grass Strong against : Fire, Ground, Rock Practice is the best way to learn and remember all of the different types strengths and weaknesses

Realmwalker won out in my Kaldheim review, but were not done with that set yet, because our best card from 2021 is #1: Elvish Warmaster Rounding out an all-Green top three (and lets be honest, an all-Elves one, too, despite only one of them being an Elf) is Elvish Warmaster