I finally managed to strengthen the wards.
Level 14 Party, Eder and 4 spell casters, one spell caster on each round stone to protect the wards from dragon attack (you see an energy shield protecting wards from dragon's attacks but not from tentacles ones).
1st round invoking every help possible
Every other round, sending either a sleep spell or using Eder attacks to stun the dragon, blocking it to call the water tentacles
You have to cast your most powerful spells to decrease dragon's life asap, but be careful not to kill him.
After 5 tries (killed him once, 3 other tries a ward was destroyed), I managed to make him lose enough life to surrender before the first tentacle attack.
Good luck.