I also just set Coalgiver as my defense.
Nice to have a way to get a set of the Valhallas, but some of the event missions are terrible - especially Arena with basics only and 150 Frontier Dungeon battles! Doing an ultimate sac outside of a sac event also feels very wrong. But yeah, we all know, they really like to make their players suffer. At least we have the option to substitute one of the bad missions by buying 1 talli with ironite if it turns out you can't or don't want to do one of the missions.
The rest is ressource draining and "inviting" players to spend a lot of money...lovely christmas time in the freemium winter wonderland. Indeed there have been times when the game was more enjoyable.
Good luck and have fun everybody!