Easy Emerald & Ruby

• Link Knights of the Round with MP Absorb
  
• Add W-Summon
  
• Add Mime

Your other characters only need Mime.

When the battle starts, W-Summon KOTR twice, then only use Mime.

• Counter + Mime, four pairs.
  
• KOTR + HP Absorb
  
• KOTR + MP Absorb
  
• W-Summon, HP Plus

Enter as only living member and W-Summon KOTR when Ruby's claws are buried. Takes about 5 minutes and a Mime or two to win.

If this isn't your last achievement and you need the AP for more than two slots (Ultima Weapon and Wizard Bracelet total 16), consult the slightly harder methods here or risk leaving out the HP Plus.