Okay. Just get Queen Grapplepot’s Pet Rescue and try to get Goblin King on stage 7 to summon.
Also, useful Mithran’s post I found.
Seems like level capped at 20 is not there anymore for spell summoning, and their summoned troop’s level will always equal to summoner’s magic (In the picture, Goblin King got exactly 25 magic). For trait summoning, it will be at the same level of original troop? Might need more data to figure it out.