One of my favourite boss battles I ran for the Murk campaign, a giant murk-corrupted elk. The party first encountered in the forest at day and spent a while following it around to a magic pool kind of thing. They stuck around to watch the sunset with it and then… uh oh, oh no, it transformed. 2020-02-09
The thing had a lot of parts, as you can see, and I can’t quite remember how it all worked. I think the tentacles were all one-hit kills but would regenerate after a turn. Unless removed with fire. I was proud of how the players handled it. They were overwhelmed at first but, luckily, the elk never left the safety of the spring it was on and the party could escape. They came back the next day for a rematch and had an amazing victory thanks to the patterns they learned.
There were two ways to kill it, reducing either the body or the core to 0HP. The body was a kind of, reliably easy enough to hit but had lots of HP. The core was heavily defended but, if they could get through to it, didn’t have much HP at all. This whole set up was inspired by how large bosses are handled in the Grandia series and I think breaking bosses into parts really solves a lot of issues with legendary actions etc. If I ever DM again I def want to play with this design philosophy.