Egg Heart

Egg Heart is the final boss of Sonic the Hedgehog 4: Episode II. The boss fight takes place in the Death Egg mk.II Zone. Eggman pilots his craft into a human heart-shaped machine surrounded by several large electrified rings. It takes 8 (console) or 5 (mobile) hits to defeat it.

Battle
The Egg Heart is surrounded by three rings, you jump from ring to ring to reach the Egg Heart. When you get to the Egg Heart you can damage it by attacking the window with Eggman in it. After the first hit lasers will start electrocuting some of the rings, you can use Tails to fly you to the Heart. After three hits some parts of the rings will break apart. After the third hit a forcefield will surround it which you can use the Rolling Combo break apart. Eggman will also charge up a gravity-based attack that will make the Death Egg dark, create a sort of black hole effect, and slam you against a wall if you take too long between hits. Repeat until the Heart is destroyed, and win the game!

Trivia

 * The Egg Heart has similarities to the Final Fever, which is the final boss of Sonic CD. Both final bosses have objects surrounding them (Electrified ring for the Egg Heart, and four rectangle blades for the Final Fever) and both have some way to shield themselves: a force field that cannot be broken unless you use the rolling combo for the Egg Heart, and the four rectangle blades for the Final Fever (although they are there from the start). Both bosses are also located in huge mechanical bases, which are the final zones for both games.
 * Just like a real heart, the Egg Heart beats during the battle.
 * There are actually two different ways to get to the center ring where the Egg Heart is. One way is to use Tails to fly to the center ring, and the other way is to have Sonic run around until the rings intersect, then jump to the next level, and do this until he gets to the center ring, but it can only be done before the rings begin to break apart.