This is a group instance that may be cleared up to seven times a week. Each run consumes 60 Game Mode Time, or combat time if you've run out. Time is only used when you clear the content. You can unlock some great headwear and cards from this each week.
Once you reach base level 45 you can access Cake Battle from the Instance Finder.
Entrance to Battle of Cake
The instance requires you to fight off waves of enemies. There are three towers that help you defend that can be upgraded using crystals that defeated enemies drop. These upgrades increase tower damage, add CC capabilities, or increase the party's defenses.
Combat time is only consumed if you successfully clear the instance and rewards are based on difficulty:
Novice (Base Lv. 45) : 1 Marshmallow
Adventure (Base Lv. 75) : 1 Marshmallow, 1 Star Strawberry
Warrior (Base Lv. 100) : 1 Marshmallow, 3 Star Strawberry
This means that even if you cannot clear the highest difficulty or are only level 45 you can still receive 7 marshmallows every week with a chance at star strawberries if you can push further. The marshmallows are the real reward here but star strawberries could luck into some decent headwar.
After your first weekly clear, you can then use the Raid feature to auto complete the other six runs. Each clear will still use 60 minutes of combat time so be mindful of this if you have a large stock of marshmallows but don't see any useful rewards for the week.
Marshmallows are exchanged for either headwear or cards from Elnie's shop and is the reason to run this content. Any headwear or cards purchased from her using Marshmallows will give you the item and the unlock/craft for it. Blueprints purchased still need to be crafted.
The headwear and cards to choose from rotate out every Monday during the reset. The instance finder is updated to show the loot for the week so you can quickly determine which items you don't have unlocked yet.
Location of the rewards list shown in the Instance Finder