The mechanic
The Bone Serpent surfaces as a heavily armored bone skull leading a train of 10 vertebra segments. Only the skull can be targeted or damaged: while the train is chained, the segments are untargetable and cannot be hurt at all, so every shot has to go into the head.
The skull is slow and armored, and its high armor blunts raw damage badly. It does heavy base damage if it reaches you, and the long body means it occupies a big stretch of the path at once.
When you finally kill the skull the chain breaks and all 10 segments come loose as independent bone remnants that march on their own, each at a slightly different speed so the train smears into a strung-out line heading for your base.
How teams beat it
Bring armor strip and pierce to punch through the skull, since raw damage barely dents it. Save your area damage and crowd control for the moment the chain breaks, when 10 loose segments spill out at once.
Like every boss, it fights inside the showdown rules: kill it and it respawns about twenty percent stronger on the rival's lane, let it leak and it crosses unescalated. The full showdown rules are in How a match works.
