I have a player who has a reputation as a bit of a munchkin and in my game he plays a budding Arcane Trickster (Wiz/Rog). He's started cooking up his own spells and already it has gone a bit sideways. His first spell was called Shocking Ray and is supposed to be a ray version of Shocking Grasp (at the same level). I allowed this off the bat but have later understood that it was a mistake for me to do that, because it allows him to do far too much damage from afar without having to close in on the enemy. Also I think he has neglected the cap on the number of damage dice (5d6 like the original spell), but I'm not quite certain. (I'll have to check up on that.)
This time, however, he wants to construct a spell that merges two existing 2nd level spells (Scorching Ray and Acid Arrow) into a single 3rd level spell. This is the result:
Acidic Ray (Sor/Wiz 3)
School: Evocation [acid]
Casting time: 1 SA
Components: V, S
Range: Short
Effect: One ray per three casting levels above 5th level (maximum four rays at level 14)
Save: None
SR: Yes
Description: One or more acidic rays spring from your hand and speeds towards its target. You must succeed on a ranged touch attack. Each ray deals 4d6 of initial acid damage with no splash. For every three caster levels you possess, the acid, unless neutralized, lasts for another round (to a maximum of six additional rounds at level 18) but then dealing only 2d6 damage. The rays may be fired at a single enemy or separate enemies, but all targets must be within 30 ft of each other.
In my opinion this is far too good on such a low level, and my second problem with it is that it merges two spell schools in a way that I don't find compatible with the rules. If the acid lingers, it should be conjuration, but then it should not have SR and also should not do this much damage. I reckon there's a reason why Acid Arrow does d4 and not d6. A friend suggested I raise the spell level to Sor/Wiz 4, but there are no good damage spells on spell level 4 and I'm betting that, too, has something to do with power levels and game balance.
What I'm looking for is not advice on how to handle the player - I know him and if it comes down to it I'll just tell him he has to stop designing his own spells. What I AM looking for, however, is advice on how to change the spell so that it isn't so overpowered and so it fits better with how other spells are designed in Pathfinder. I've read the chapter on spell design in Ultimate Magic, but without clear cut rules I'm honestly pretty confused.