sur gamefaqs ils coseillent d´etre au moins au niveau 7-9,pour le battre et si tu comprend un peu l´angais leur stratégie poura peut etre t´aider :
Strategy:
The Sentinel will take a long time to wear down. He has a lot of HP for this point of the game. The easiest way for me was to use your newly acquired Purple Moon Stone and level up with Vyse equipped with the Purple on his weapon. At least until you get the Crystali (If you are diehard, like me, you can go for the second attack spell). Once you have that, and Aika has some healing spells(or should) you are ready to battle him the easy way. When you first encounter him, you should have enough Spirit to use Crystali at least
once on the first turn. If you only decide to do it once (or don’t have it on more then one character) then focus with the second character. It should do from 100 to 200 points of damage with each casting. With your second person focusing, you should gain SP fast. Cast Increm on Vyse with Aika ASAP. When the boss uses Search (With the red beams of light) then he is ready to use his
ultimate attack. Be warned of this, since it takes 2 turns to power up, it is extremely powerful. Make sure your HP are greater then 300 and maybe even cast Increm on Aika just in case, and be ready to heal. Also note with this boss, you have a big advantage. If your level is high enough then he will always be the last to attack, making it easy to heal.