Mega-emperer easy kill:

Make sure before you battle him you have all the parts for the laser, break and missile cancels. Set your three bots to have one of each part and any legs you like. Now the only way he'll do damage to you is with medaforce.
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Code Effect  -  Hold Select, press Start (on title screen) Test Mode (listen to BGM/SFX, erase data)

No Random battles  -  Once you can use the 'Mini S. Wheel' Vehicle, use it anytime your not in a building to avoid Random battles.

Start a medafight, build up a medaforce. next use the medaforce: Schrapnel Attack so all parts will damage to half, to defeat a medabot easily.

Test Mode  -  Hold Select, press Start on the title screen.

Anti-Air  -  Medabots with Air-Type Legs (A Lightblue icon covered with a Medabot with wings) are very weak against Anti-Air.

Anti-Sea  -  Medabots with Sea-Type Legs (A darkblue icon covered with a Medabot with fins) are very weak against Anti-Sea.

Beat Mega Emperor  -  Metabee's Parts:

HEAD: Missile
RIGHT ARM: Sweat cloak (to Counter a few attacks)
LEFT ARM: Arm Shutter (to defend yourself against Missiles and Napalms)
LEGS: Ochitsuka

Use Metabee's Sweat Cloak to prepare a Counter to send all attacks from Mega Emperor back to him, with a major Damage result.

Also give one of your medabots (Partner 1 or 2) "Destroy" Parts to get rid of those monsters.

Listen to the music from the game - Press Start + Select at the title screen.

Schrapnel Trick  -  Required: Mermaid Medal (lvl 30 or higher) or Ghost Medal (lvl 60 or higher. Make sure your far enough in the game.

Equip the parts with the most Armor:
HEAD: Clear Armor
RIGHT ARM: Right Guard
LEFT ARM: Great Shield
LEGS: Burly or Chauffeur.
equip those parts to a male tinpet, but also the medal above.