I beat the game a couple days ago, might as well put down my thoughts on it.
I beat it with everyone at lv13 or higher, average level 15, quite handily. The first part of the battle is a cinch, the second part requires a lot of patience (it took me close to two hours on the last battle, all three of them). Only one character was level 20 (obright), and two above level 17 (main character and dantares).
The Hero's Test was a great place to level up people I never used. For instance, Kahn (who I promoted and then never used again) was at Level one, and by the time I left the hero's test, he was at lv 11. Same with everyone else. Even though the enemies are easily defeated, if the character has never fought them, the character will receive all 49 exp for them, up to level 10. Made my life very easy. Same for the 2nd and 3rd parts of the test, except it'd only go 1 or 2 levels before being only 1exp per kill.
As for the 2nd battle, It took me near 2 hours because I kept experimenting to see what the computer AI would do in certain situations. Sadly, the AI is kinda dumb.
I used my archers w/ longbows to attack the two mages on the side (One at a time) - rather than destroy the archer, they'd heal each other w/ only aura lv. 1 (they're capable of aura lv3, I think). After they're dealt with, the boss is insanely easy (since it is the very last battle). The AI wouldn't attack unless it was being attacked (strangely enough) so when it had the chance to use a Photon attack it opted not to. I ganged up on it and attacked it with all my characters, and after two turns (magic attacks mostly) Arrawnt was finished. The hero's test for the Tiamat Rod was harder.
Speaking of which, the Tiamat rod is awesome against Gigabreakers. They were taking 70+ HP a pop if it wasn't outright killing them. I equipped the Rod w/ Noon.
Essentially, Mithril weapons are awesome, as are mithril accessories. The Ninjas did especially well with them, since they were almost always attacking w/ special attacks. I used the weaponcraft guide at Gamefaqs - it was a major help in making the perfect weapons.
I couldn't get Fynndig to be of any use (starts at level 15 w/ a low attack) so I used him as a decoy, since the AI will attack him before anyone else.
As an aside, I didn't forge many friendships, because I don't care much for them. I have all the secret characters, including Pen. If you want to beat the final battle a little faster, don't bother getting Pen - I was so fed up with him I let him die purposely. I think the idea was he'd be like Domingo in SF1, but I for the life of me couldn't get him to be useful at all. Maybe after lv 20, who knows.
That's all, I guess. I wish I could read Japanese for the other scenarios...
