Get your green bar full either by powering up using [L], or by blocking attacks. When your bar is full and your character starts flashing, press [R]. If the attack was successful, your opponent/opponents will die.
Enter the "EkiTa" as Emeryl at the Sonic Team building. Equip technique as ground power. Let your opponent come close to you, then press [R]. He should attack furiously, then end with a whip-like attack.
Clear Emerl's episode in Story mode. Upon completion, Green Hill should be available as a stage in Battle mode.
To do a combo, it is best to start with an upper attack. When the enemy lands, do your "PPP" or "PPK" combo then a heavy attack.
After you defeat Dr. Robotonik as Emerl, you will beam over as Sonic and must defeat Emerl when in a turbo-mode state. It does not matter how weak Emerl's attacks might have been, he can still whip you around like nothing. Use your special attacks by pressing [A] in the original order (1, 2, 3). When he comes at you. jump and press [R] until he is KOed, then just press [R] when he comes at you the next time. If he does come back at you rather quickly, he is healing himself; run at him and press [R]. Keep doing this pattern and you should defeat him easily. Note: This works 99.9% of the time.
After defeating Dr. Eggman as Emerl in his story mode, you will have to fight the now out of control Emerl. The easiest way to do this is to use your basic three hit combo, then hold [Back] and pressing [B] to hit him up into the air. When he comes back down, hit him back up into the air again. Keep doing this until you win. It helps if you do the attack just before he touches the ground.
Skill points for Emerl
Get an Emerald: 10 skill points
Finish a battle without taking a hit: 5 skill points
Finish a battle without being KO'd: 2 skill points
Win a battle: 1 skill point