Sonic VS Shadow
From Sonic Retro
There are two Sonic VS Shadow boss fights in Sonic Adventure 2. Both fights are in both stories, and happen at the same points in the story.
Sonic can also fight Shadow in the game's multiplayer mode, as a 2-player versus match.
The arena is themed to Green Forest, as Sonic runs off to said level after the fight, while Shadow enters from White Jungle, its counterpart. It is a fenced-in grassy platform with a indestructible box in the ground, similar to the breakable boxes covering holes in the main stages of both characters. Most noticeable differences are the fences, the absent platforms, and the water being at the same height as the platform itself, being far below in the actual level.
Both Sonic and Shadow use the same attacks. No special attacks. The only thing you have to worry about is falling off the small arena, so be careful. Bear in mind, if you deflect an attack, the latter will not lose health.
After the Battle
Once you win the fight, Eggman tells Shadow to get off the island, as it is going to blow up. Sonic overhears this, and Shadow and Sonic split up for the time being to escape the Island. On the Hero Side, Sonic traverses through Green Forest. Because Shadow went through White Jungle beforehand, he races to the safe to save Rouge and the Chaos Emeralds she procured.
The fight is very different from their first encounter. The fight takes place on an endless highway, with the road behind them constantly falling, forcing them to stay at top speed for the entire fight.
The level is themed to Final Rush/Final Chase. It´s a straight road collapsing behind the hedgehogs, forcing them to run forward or risk re-entry. There are also meteorites strewn about space, giving the level more detail and injuring players who collide with them.
Both Sonic and Shadow use the same attack, but with different names. The floor continuously falls behind you; think of Sonic and Knuckles's Great Eggman Robo's second phase with the falling floors. If at any time the boss character falls too far behind the player, he will return to the front using Chaos Control.
The best way to attack this boss is to use the Homing Attack, Spin Dash, or Light Dash right into the boss. The best time to attack is when the opponent is stopped, either from re-spawning with Chaos Control or pausing to use their special move. Abusing the AI by intentionally falling back, then rushing forward when they pause to strike with a special move, is a risky but effective strategy.