Strategy & Tips
Jackenstein is an unusual and unique boss encounter, fighting him through traditional means is impossible due to the overwhelming darkness of the area preventing you from targetting him. Making progress in this fight is entirely dependent on building large amounts of TP and using it to perform the Unleash ACT, which allows you to build up progress towards using SPARE on Jackenstein. While this seems straightforward, the darkness of the area does more than just make targetting Jackenstein impossible, it also heavily reduces the amount of TP you gain from actions such as GUARDing or through grazing attacks. This is because TP is instead gained through Jackenstein's unique battle gimmick: Treasure. Each of Jackenstein's attacks is actually a maze shrouded in darkness, the exit is unlocked by acquiring a key and along the way you can pick up treasure, which increases your TP by 1 every time you collect it.
Simply navigating the maze and collecting treasure would be fairly easy, and likely wouldn't require a guide on its own, which is why we should acknowledge the major, lurking threat in each of Jackenstein's attacks, his impatience. Once you acquire a key and after a short amount of time passes, a large, red pumpkin will appear and announce that "YOUR TAKING TOO LONG" (Demonstrated above.) before chasing you through the maze. Failure to escape in time will result in every party member taking some pretty hefty damage, so memorizing the layout of the maze on your way in can be helpful as you'll be forced to make a hasty retreat. You'll notice that the chase does not begin until after you've acquired the key, so if you find yourself in need of TP and haven't collected all of the treasure in the maze yet you're actually free to take as long as you like retrieving it so long as you don't pick up the key. It seems Jackenstein is more concerned with the property theft itself rather than how long you took to do it after all. Defeating Jackenstein is a mandatory part of Chapter 4's progression, and after doing so and befriending the gentle giant he will provide you with his Claimbclaws, which will allow the player to navigate terrain they previously couldn't.