The mechanic
The Sundered Sentinel arrives as two armored statue halves walking side by side, each a substantial armored target in its own right. You have two bosses to bring down at once from the very start.
When the first half dies, the survivor absorbs it. The remaining half gains 20 percent damage reduction and its speed rises to 1.3 times its old pace, becoming the single hardest lone target in the race. Killing one twin therefore makes the other tougher and faster, not weaker.
When the empowered survivor finally shatters it bursts into 6 fast rune shards that scatter for your base as the last wave of the fight.
How teams beat it
Consider damaging both halves down evenly and finishing them close together, so you spend as little time as possible against the tougher, faster survivor. Bring armor pierce for the halves and save area damage for the 6 rune shards at the end.
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.
