Robotnik's battle in Knuckles the Echidna's Appartment Zone is very hard. He faces Knux at the end of Act 2.
Robotnik pilots an enormous mech on the right side of the field. The left arm when placed in front of Eggman's glass dome will attempt to crush Knuckles. The right arm when placed in front of Robotnik's glass dome which will shoot lasers, rockets, and bombs. The only way for Knuckles to damage Robotnik's glass dome is to disable the arms. Both arms have a glass dome on themselves, and when a dome of an arm is destroyed, the arm is disabled. Once the arms are destroyed, it is much easier to reach Robotnik, but he can now shoot enormous lasers (that charge up) and is capable of lifting up his mech to crush and kill Knuckles. If Robotnik is hit enough times, the machine will blast apart and reveal a core device that Robotnik is piloting. In this final form, Robotnik will bounce all around the field uncontrollably, destroying bits of ground he lands on. Knuckles must damage Robotnik now by gliding into his machine to drive him into the spiked walls. If Knuckles does not succeed in time, there won't be any ground left. However, if he does succeed, the area blows up and Knuckles wins.