I've been DMing 1st edition for about 12 years now, and while I've experimented with different editions I always go back to 1st. Over the years DMing I have written a number of my own house rules to fix the many balancing issues 1st edition has. But, there is one spell/mechanic that I have yet to find a good way to balance, and that is the Magic User/Illusionist spell phantasmal force.
For magic users the spell is a 3rd level spell (can't be used until the magic user reaches 5th level), and generally speaking my players who play magic users don't tend to rely on it too much and the power level of the spell doesn't feel overpowered when you have other spells of the same level like fireball or lightning bolt.
The problem comes with players who play an illusionist. For illusionists, phantasmal force is a 1st level spell. My groups have gotten smart enough to know that at least one person should play an illusionist and memorize as many phantasmal forces as they can each day. Very rarely do my player illusionists use other illusionist spells. The reason why is that phantasmal force is such a powerful spell if used correctly (deadly illusions are fatal to those who believe it). The most common illusions I see from it are a continuous barrage of illusory fireballs or magic missiles, which can clear out entire hordes of dumb enemies who believe it. (To increase the chance that an enemy would believe the illusion, my players like to do a trick where they have the magic user cast the real thing, then have the illusionist step up and continuously replicate it. Since the enemy has been hit by the real thing they have no reason the disbelieve the fake one.)
Given the rules for the spell:
Phantasmal Force (Illusion/Phantasm)
Range: 8″ + 1″/level
Area of Effect: 8 square inches + 1 square inch/level
Components: V, S, M
Casting Time: 3 segments
Saving Throw: Special
Explanation/Description: When this spell is cast, the magic-user creates a visual illusion which will affect all believing creatures which view the phantasmal force, even to the extent of suffering damage from phantasmal missiles or from falling into an illusory pit full of sharp spikes. Note that audial illusion is not a component of the spell. The illusion lasts until struck by an opponent — unless the spell caster causes the illusion to react appropriately — or until the magic-user ceases concentration upon the spell (due to desire, moving, or successful attack which causes damage). Creatures which disbelieve the phantasmal force gain a saving throw versus the spell, and if they succeed, they see it for what it is and add +4 to associates’ saving throws if this knowledge can be communicated effectively. Creatures not observing the spell effect are immune until they view it. The spell can create the illusion of any object, or creature, or force, as long as it is within the boundaries of the spell’s area of effect. This area can move within the limits of the range. The material component of the spell is a bit of fleece.
What are some ways I can modify the rules or reinterpret the rules to limit its power and stop players from overusing it, while at the same time not limiting the power so much that no one wants to play an illusionist?
The only thing I've done so far to limit the power of the spell is to "fudge" saving throws for targets. So, in a case where the spell would normally clear a room of enemies, I usually make sure at-least 1/4 disbelieve the spell (or simply weren't paying attention) and survive just so the remainder of the PC's have something to do.