44/45 Defeat any event monster 10 times I have been told that it doesn’t have to be the mythic event monster.
Yes. After you have unlocked Mythic room, any monster counts towards this goal.
BUT you do need 50 Mythic cards to level to level 5 anyway (so you have to defeat Mythic monster 10 times if want to make sure to complete the last goal 45 - unless you get it from Mythic Packs/Event shop). Earlier, completing this goal leads to change in overall ranking of about ~20.