How to beat Octogon, the Mermaid's Cave boss (Dungeon 6). Dual-depth loop: surface sword windows between rock volleys, then dive with the Mermaid Suit to slash the soft underside. Claim Lonely Peak. Instant answer below; the checklist tracks prep through victory.
Depth loop Prerequisites Form cards Stuck diagnosis Phase shortcuts Step checklist FAQ
Memorize the surface ↔ dive cycle before the boss door opens. When damage stops, Octogon changed layer — follow it.
Mark gear and story progress you already have. Saved in this browser only.
What each depth layer accepts for damage, and how mermaid controls fit in.
Match what you see in-game to a cause and a fix.