With Metal Sonic knocked out of the fight, the player is transported to the another hall that leads to Nack the Weasel, who briefly shows up in the Zone. Presumably in the Atomic Destroyer Zone to steal the final Chaos Emerald, which is in Dr. Robotnik's possession, Nack turns and flees after catching sight of Robotnik's armored pogo-mech, leaving the player to face the Doctor one-on-one.
In battle, Robotnik can only bounce back and forth in his pogo-mech. As such, the player can easily hit the mech many times. The only thing the player should watch out for is getting crushed by the mech. After ten hits, Robotnik quickly moves away to the next room, where doctor is waiting the player on his another vehicle to be encountered.