I am a huge fanboy of Summoners. I just... love them so much. And by skulldude... You mean Stalfos?
All Summons summon spirits of past enemies. The idea is to use this magic against your opponent, and it acts as a "haunt". Not all summons are offensive, for example: Skulltula I is a defensive summon, which blocks a projectile (Archers - die.) It only lasts one projectile, but may give enough time to regenerate enough MP to summon again :/