Gotta say, it's grindy as heck to get the unique items from challenge spire, a RNGfest, it's egregious. Around 40 runs from the fire challenge I got just 4 uniques out of 14.
Nature challenge got to be the worse. You have to fight 3 challenging boss, the difficulty isn't the issue, the issue is you have to fight them multiple times. One reason being just to get a hint to make a type of item, and second to get some random material to make said items. But that's not all, you need said items to catch fishes because the recipe need them too. Then on top of all that, after you scratch your head to figure out the recipe (imagine going without walkthrough, I can't, it's frustrating), you thought it's some easy peasy input right material and you get strong/unique weapons/armors? No no no, that's easy, once again it's gachafest, you'll get some random stuff.
Bonus challenge, where you can ask Kiki a harder challenge, a 3 battles in a row where your opponents change on 2nd and 3rd fight. It seems you can get a unique weapon through this, but only if you get to fight all 3 challenge boss on the 3rd fight, and again you need to be lucky to have them appear as opponents (as it's random who you may fight).
All these mechanics are just plain bad time sink, rather than a fun added difficulty.
Nature challenge got to be the worse. You have to fight 3 challenging boss, the difficulty isn't the issue, the issue is you have to fight them multiple times. One reason being just to get a hint to make a type of item, and second to get some random material to make said items. But that's not all, you need said items to catch fishes because the recipe need them too. Then on top of all that, after you scratch your head to figure out the recipe (imagine going without walkthrough, I can't, it's frustrating), you thought it's some easy peasy input right material and you get strong/unique weapons/armors? No no no, that's easy, once again it's gachafest, you'll get some random stuff.
Bonus challenge, where you can ask Kiki a harder challenge, a 3 battles in a row where your opponents change on 2nd and 3rd fight. It seems you can get a unique weapon through this, but only if you get to fight all 3 challenge boss on the 3rd fight, and again you need to be lucky to have them appear as opponents (as it's random who you may fight).
All these mechanics are just plain bad time sink, rather than a fun added difficulty.