The Gigas Shadow is a Pureblood Heartless that is found in Kingdom Hearts Final Mix and Kingdom Hearts 358/2 Days.
A Gigas Shadow has a vaguely humanoid body structure. It has a round, spherical head with circular, glowing yellow eyes. It also has two long, twisted antennae sprouting out of the top of its head. Each of its hands has three clawed fingers, and its feet are large and lack any distinguishable digits. With the exception of its eyes, this Heartless's body is completely pitch-black. The Gigas Shadow is identical to the basic Shadow and Mega-Shadow Heartless, but is much larger than either of its two relatives. In Kingdom Hearts Final Mix, they are simply normal sized Shadows that gradually grow to their limit, possibly a reference to Sora's use of the shrinking potion.
Kingdom Hearts Final Mix
Gigas Shadows only appear after many waves of Shadows have been defeated in the Bizarre Room. They attack the same way normal Shadows do with the exception that they do not become flat and move along floors. They are large and have much more HP than other Heartless in the area. They can quickly surround and swipe at Sora but this can be avoided by moving into a corner on an elevated surface where only a few Gigas Shadows can attack at once. If a Gigas Shadow manages to strike Sora, it will escape the fight, reducing the odds of winning a rare Synthesis material.
Kingdom Hearts 358/2 Days
Gigas Shadows are relatively easy to defeat. They are only capable of a claw attack, a jump attack, and becoming flat and moving along the floor. The easiest way is to block their attacks. Blocking a claw attack will do nothing but blocking a jump attack will stun them temporarily, giving you time to do a combo. Repeat this enough times and the Heartless will fall easily.