How do you defeat the final boss in dragonquest ix

It's best to attempt to beat him at a level that's 45+. If you're still in your late 30's, then you're seriously underleveled. I suggest you go to the Oubliette (good monsters, priest to save your game and healing pedestal).
You might want to get lots of skill points then spend them on the Paladin's Virtue tree (defense, HP and resilience!).
You should also equip Life Bracers on your party members. They heal 20 (or 25. I forgot.) every turn, so you don't have to put all the weight on your healer.
Beware of his Disruptive Waves! They will cancel out any of the tension you've builded up AND take out the good status effects!
Make your healer do Care Prayer (if he/she has it.) on the first turn. That way, his/her healing capabilities will improve. Also, make them use multiheal often. One of Corvus's attacks can do 150-170 damage on ALL of your party members.
There are more tips, but these are all the ones that I can remember.
But if you REALLY wanna be sure that you can beat him, get your party up to lv. 60 or over. ;D