i found out my self the way to level up and get mana for skulls.

first go to a low level area with alot of monsters.

second you need alot of Sp items (to cure your Sp when it get low).

third you need a healer around to help you.(just in case you take damage)

fourth you need to try to use the best spells you have.(only if you have alot of Sp)

if you do all of that your skull will level up higher, have more mana, and you'll be swimming in hell.(money)