Summons gain experience every time you use up one of their summon drive bars, and the experience can be tracked in the status menu. Forms level up differently. For each one you must meet specific requirements. Valor gets experience every time you land a hit, Wisdom gains experience every time you kill a heartless, Master gains experience by collecting the gold orbs such as in Mulan's world whilst in the form, and final form gains experience for every nobody you kill. Hope this helps.