After you clear the Tower of Mirrors (and if you've collected every Dragon Pearl in the game), go to Dragonlord Castle and turn in all your Dragon Pearls.
Once you've traded 40 Pearls, you get the Dragonlord Card, and the Dragonlord tells you this is the last time you'll see him in his current form.
Now, if you leave the castle and come back, you can fight the Dragonlord!
All I can say about this battle is, be prepared to watch a lot of videos...
I used Zilva, Tink, Temi, Lufa, and Blont.
First, have Zilva and Blont cast Reflect on each party member.
Have Lufa cast Dark Matter every turn. (It lasts a few turns, but it's easier to just keep it going than to watch for when it wears off and cast it again.)
Have Temi and Tink use Gigavolt as often as possible.
Once Reflect is on each party member, has Zilva use his regular attack, and have Blont either attack or use Poison Arrow (you're not likely to poison the Dragonlord, but Poison Arrow does do some pretty good damage).
Pay close attention to everyone's Reflect status, and to their MP, and make sure you use Strange Grass before their MP runs out!
When you finally do beat the Dragonlord, he'll tell you how many turns it took you to win, and what your highest damage was.
Now (as far as I know) you've done everything there is to do in Tenmilli RPG.