the kill 25/45 event monster quests need some heavy grinding. i used to buy gems but not anymore after the recent update, not giving them my money. i use the gems to complete the gold quests. then you need some luck for the get 15 event mythic card quests. normally if i dont get the event mythic cards from the event chests or the butt pass mythic chest, then i wont be able to complete the event.