YMMV / Astlibra

• That One Boss:
  • The boss of Chapter 3, the God of Fire Agni. For starters, you can't damage him directly, as he's surrounded by a circle of fireballs that can damage you by contact. And no, hitting his body won't work either, as his weak point is his eyeball at the center, which is protected by the aforementioned fireballs. But that's not the worst part. He's also constantly shooting fireballs at you, and can also inflict the dangerous Bleed status. The protective circle of fireballs can be destroyed, but they all have high HP (and also don't display their HP even with the HP-display skill equipped), so this can take a while, and the only way to damage him otherwise is by either using a staff or skill that can penetrate through the fireballs and hit him at range or shield dashing into his eyeball and whacking at it (which is very risky). And if you do destroy the shielding fireballs, he'll switch to an altered moveset and start spamming the arena with fireballs coming from the ground and Agni's Fire at the same time. Be sure to bring enough healing items, as unless you can hit him through the shield, this can be a very long battle.
  • The fake Demon King in chapter 5. The boss itself isn't a problem, being slow moving with attacks that are easily dodged or blocked. The problem is that it constantly summons additional enemies, including other bosses. And sure, the ones from chapter 2 aren't going to do more than interrupt your actions, but that's still a big issue (good luck healing without abusing possession skill i-frames), and he's not limited to those. He also summons enemies from chapter 5, including the wyvern boss you fought just a little while before. And he can summon multiple copies of each boss simultaneously. The only upside is that so many enemies means you'll charge st for possession skills quickly, but even then, this fight can rapidly turn into an absolute mess where your only option is to spam possession skills and hope you don't get creamed in between.