So I got him to join the party, but he is only level 40 while rest of the team is level 68+ Is there any point in training him, is he vital to the late boss battles?
That's what happens when you grind too much, probably on Tokotokos. A friend of mine is like 78 at this point in the game. I was 53 and had no trouble getting to that point.
If you find him useless it's your own fault for overleveling.
Honestly, I find him way more useful than Swaine because Swaine's MP pool is garbage, plus Marcassin has healing on himself so you don't need to give him a healing familiar to be able to heal people. --- It took hundreds to kill me but I killed humans by the thousands. I am sublime! I am the true face of evil! - Luca Blight