I had the exact same freaking situation when facing the lizardfolk boss for the first time, and I found a way to win that fight despite the incredibly strong defenses. This time, though, the situation was quite tougher, since his shell was almost completely covering his body, leaving only small cracks here and there.
But that was actually something I could work with. All I needed was to time my attack with the movement of his defenses, and judge the angle correctly, which was making it very difficult, but not impossible.
I tried few times, using the skill, and making my blade pop out on the other side, passing through The Void, and failed. He realized what I was doing very quickly, and made the crystals start moving around him, to make my tactic even less likely to work than before.
- Fuck you - I said, as I jumped backward to gain some distance.
- Rahahahaha! - he responded with a laugh.
