I don't know if it's a bug or what but I can't complete Stage mission 21. I have problems with either the 4 dinosaur monsters summon in one duel (using Gore vs Soulburner) and the one in which you have to summon LINK 3 monsters 2 times in a duel as said by others in the comments. Anyone managed to pass this?
Completed those yesterday, I am on stage 26 now, All i did for the Gore vs Soulburner one is load a deck with just the monsters from Tyrannos deck, and it completed just fine for me. For the link summon 3 times, i think if you resurrection link with salads it will not work, i had to hard link, code talker and girsu.