The mechanic
The Thunderhead is a slow, armored storm colossus. It grinds up the path from the front like any boss, but its signature trick attacks you from the wrong direction.
At 75, 50, and 25 percent health it calls 3 sparks, the fastest enemies in the entire game, that spawn at the path entrance behind your towers rather than in front of them. That means a second front chasing your base from behind your lines, where most of your towers are not aimed.
On top of the sparks it summons 1 slower arc elemental every 12 seconds, and when the colossus dies it splits into 2 more arc elementals. You are constantly managing pressure from both ends of the path.
How teams beat it
Keep at least one tower positioned to cover the path entrance so the sparks spawning behind you do not run straight into your base unopposed. The sparks are unarmored and fragile, so a little crowd control or splash near the entrance shuts them down before their speed matters.
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.
