Looking at most of these videos they were mostly done with 40-50 average teams or lower. If you happened to get your units to 50CC20 you should be golden for this event. However, if you didn't you probably wondering on what units are the most important. Once again looking at the video it was fairly clear you wanted at least two units to fight the Dragon Mages(Princess, Bandits, Dorania, or Valkyries), you wanted a couple of strong healers or a lot of decent ones, and finally one or two Mages to deal with the rush. Those are the units you should focus on to get you through.
After this you'd want to focus on Witches, Archers, a Heavy Armor and a Soldier which will make it even easier for you to beat all these maps. If you figured out how to deal with the Ryujin King map from Ryujin Invasion you should be completely okay up until the last two maps.
Honestly though the side rewards are alright. Four guaranteed silvers, A gold(Which is useless since we have Katie and Aria), platinum fairy(which I imagine those who could obtain Dorania last event won't need all that bad), Black fairy(yay free black fairy. fairly hard to come by for most players right now), and Rainbow fairy(gives the chance to increase the skill level of the unit it is combined into). I'd honestly recommend saving that fairy for Karma. Better her then what most people have right now.
Other then that... I'd say farm the ever living daylights out of this guy right here
Well... that is if you have...
Thetis, Sakuya(potential new unit), Hina(new unit), Victoria, Adele, Despara, Kerry, Garrett, or Cloris... Yes I know some of those we all get. The reason you want to farm the daylights out of him is because he has Increase Attack 3 and 4. All these units have this skill so you'll want to get them as high as possible if not 5/5. It really is the only thing to do after you get the completion units.
Note: You need to save at least one of the Bronze and Silver Dragon Soldiers to unseal Anya.
DO NOT FORGET THAT
Note 2: This is the only time you'll have to get the completion rewards. If we get this as a revival event in the future they will not be there.