Hotaru Hi-Watt is the sixth mini-boss fought in Sonic Mania, encountered at the end of Stardust Speedway Zone Act 1. It resembles a giant version of the Hotaru badniks found throughout the Zone, several of which appear to aid the boss during the fight.
After introducing itself, the boss will fly off the screen and summon a smaller Hotaru to fly around and zap the player with lasers, similar to the ones fought in the third Act of Metallic Madness in Sonic CD. Once it's destroyed, two more will fly in and co-operate to create an electrical beam, then drag it down to attack the player before flying away. If one of them is destroyed, it'll break the circuit and stop the attack.
The boss itself will then return and fly from one side of the arena to the other, firing a massive laser into the ground. At this point, the boss is vulnerable to attack from the player, but they must take care not to get hit by the laser. Afterwards, the attack pattern will repeat from the beginning. After six hits, the boss is history, allowing the player to travel to the future for Act 2.