Extra missions are a good way to test your skills. There are various challenges which one can take. For example you can kill a certain amount of monsters or pass through a maze in a limited amount of time. Personally I'm stuck in one of the Cait Sith missions in whcih you have to kill a ceratin amount of monsters. Unfortunately Cait Sith doesn't have any long range attacks and his HP is too low. Sometimes it is difficult to attack without getting hit.

i haven't gotten very far in the extra missions. Im stuck on the one in the wasteland where ur on a turret and have to kill monsters and rack up 500 points . I think it's the 8th mission. ???

For that one, I used the turret on the lower level. Then, in the last few minutes, I equipped the Ultima Weapon and killed the monsters as fast as I could.