Hi, I just find a way to train Tys to lvl 99, time consuming though.
There are 2 tricky parts here:
1. Tiana and Luke can gain exp from hitting allies by Mass Attack. With evelin and sarah's heals, these four cheaters can gain exp much faster than killing enemies. My Tys kills almost all the enemies (and with amptitude!) after meeting the Goddess and she is still 10+ levels lower than those 4 guys. After the stage "A Wail of the Dragon", my Tys is level 84 and those four reach level 99.
2. In the stage "The Thunderbird", Luke is unbeatable, which means his health can never go below 10. Since he is already lvl 99, every hit to him will gives bunch of exp and this will only ends when he fights Tiana. So at the start of the stage, everyone is moved away from Luke except Tys. Then it's a 1v1 duel between them:). With my lvl 84 Tys, you can do this with only few save/loads. (outburst Luke only have 50% chance to hit Tys when her will is only 90,with Full Force and Counter though. Later when Tys' Will rise up and Luke's will drop to 150, Tys can fully guard all Luke's attack except Dragon Strike).
Then lvl99 Tys! :D
She will get all the combat abilities at lvl 94, and MAX all the skills at lvl98. (lvl 5 Aptitude at lvl98 lol) She one-shot every boss except Ascard (not sure about Deux Ex Machina, but at least should be easy since she one-shot Celtakroth before his Mass Attack :D) However, she is not the best for Ascard fight because of the low dmg of her final weapon and no bonus from field. She can do 7k dmg to Ascard while lvl99 Luke can get over 10k, though Ascard cannot hurt Tys at all:)