The Death Machine, also known as Death Mech in the GBA release, is an optional boss exclusive to the Game Boy Advance and Complete Collection versions of Final Fantasy IV. It is fought in Cid's Trial.
It uses four attacks in the same pattern - Flame, Laser, Piercing Laser, and Object 199. Laser and Object 199 are virtually instant kills, but Flame and Piercing Laser only do moderate damage and can easily be healed.
In addition to exploiting the Death Machine's infrequency of attacks to get more damage in, it is rather easy to defeat as long as one has Rosa or Porom in the party to heal. Defeating this high-level monster will result in a special weapon, Thor's Hammer.
An easy way to make use of Cid during this battle is to have him equipped with the Mythril Hammer, which can be bought in the town of Mythril, due to its innate ability to do extra damage to mechanical enemies.