Make sure you complete all the quests, since each chapter, quest, and boss awards Crystal ×50 on first-time completion. Complete the loyalty character's loyalty bar to keep her permanently, which also unlocks her Fate Episode for more Crystals.
Obtain as many Damascus Crystals from the event as possible. They are a good long-term investment for the future. There are 3 that you can buy from the event shop. You need 10 Damascus Crystals to create 1 Damascus Ingot.
Bellringer Angel has a one-of-kind Call effect which can reset the cooldown of another random summon, but only when 3★ uncapped. Compared to other event SSR summons, Bellringer Angel may have long-term potential. There are only four copies of this summon in this event. It does not drop from any of the event bosses. Unfortunately, they are only rewarded for clearing Very Hard, Extreme, Maniac, and Nightmare Level 60 battles, some of which can be very difficult for newer players.
Luna: Buying event-exclusive characters is generally recommended even if you don't plan on using them. You will at least get Crystal ×50 for completing her Fate Episode.
Silver Bow has a EX big boost to Wind ATK weapon skill. However, buying more than one copy from the shop requires Hellflame. These only drop from Nightmare difficulty. The weapon itself is decent filler, but not remarkable. Try and get one if you don't have a Wind EX (aka Unknown) weapon. Otherwise, it's up to you whether it's worth getting a 3★ copy.
Luna's Friend has a EX big boost to Dark ATK weapon skill. Staff-type weapons have the lowest ATK stat in the game. Regardless, newer Dark players can still appreciate a filler Dark weapon while obtaining Celeste weapons. Buying more than one copy from the shop requires Hellflame which only drop from Nightmare difficulty. Try and get one if you don't have a Dark EX (aka Unknown) weapon. Otherwise, it's up to you whether it's worth getting a 3★ copy.
This challenge is meant to be a puzzle. You can figure it out yourself or read on for the answer.
Mordecai will revive himself to full HP unless you use a Chain Burst as the final blow. Use Arisa's Fairy Circle for 3 Fairy stacks and Luna's Soul Conversion for 3 Shadow stacks. When Mordecai reaches roughly 25%-29% HP, use both character's Charge Attacks. The Air Detonation Chain Burst should defeat him. Make sure you do not kill him with the second Charge Attack. Otherwise, you need to start over.
Luna's Soul Conversion does not require knocking out other allies for her to gain Shadows.