Dr. Bokchoy
Head north and then east as soon as you start the level "Jungle A." Use a strike Medapart to knock down the bridge. Press the yellow floor switch and go through the door that opens. Keep walking up the path until you end up in a robattle. Fire head missiles at Dr. Bokchoy or use medaforce repeatedly. Stick to this plan and you will easily defeat your opponent.
Dragonfly
Equip Metabee with the fire arm and stand on the four green buttons in the "Laboratory A" level. Go north into the circular room and dodge the boulders. Push the yellow button in the west section then go to the east side and open the doors. The most important thing to do in the robattle with Dragonfly is to dodge his gravity attacks and sword arm attacks. Fire when he stops attacking and you will eventually beat him.
Cordy
Avoid the laser turret in the "Ruins-Out A" level by staying close to the wall to the north east. Follow the large wall when you encounter it and then the wall after that. Walk north and then east after the locked door to find a robattle room. Enter the room and Cordy will be waiting. Stand on the two green switches to start the fight. Watch your step as there are sections of poisonous floor. Charge up your medaforce and try and hit him with it. He's extremely quick so you could miss, but there is a backup plan. Repeatedly fire missiles and rifle shots to wear him down and eventually defeat him.
Maxsnake
Head north from the indoor garden in "Ruins-In A." Turn west and then north, sticking to the path. Push the green button and head back to the point that you turned west. Choose the east option and enter the door, which wouldn't have been open before. Go through and head north for a robattle. Maxsnake will hunt you down no matter what, so use this against him. Run to the end of the room and he will follow even though the eight cannons along the way are firing at will. If he survives this onslaught, finish him off with your rifle.