Saeviomagy
Adventurer
The spellcaster could have used any number of other spells to achieve the same effect (ie - a disarmed opponent). True-strike/disarm is probably the most likely to work, but conversely it also has the most minimal gains (it took you two rounds after all and the guy can just pick up the weapon again).
Casting grease is less likely to work (ie - he could make his save), but has worse consequences (he keeps having to make saves or drop the item).
Casting shatter is again less likely to work, but has far worse consequences.
Unseen servant could be used in conjunction with a normal disarm attempt so that if you're lucky, the weapon is out of reach, not just on the ground.
Etc.
Casting grease is less likely to work (ie - he could make his save), but has worse consequences (he keeps having to make saves or drop the item).
Casting shatter is again less likely to work, but has far worse consequences.
Unseen servant could be used in conjunction with a normal disarm attempt so that if you're lucky, the weapon is out of reach, not just on the ground.
Etc.