Q:

How do I overcome Regen X/Good as a wizard, in Pathfinder, short of summoning a celestial with a good aligned weapon?

Some of the evil outsiders in Pathfinder have a regeneration that can only be negated by 'good' e.g. damage from good aligned weapons, our natural weapons of good aligned beings.
How do you overcome that as an arcane caster?
Is there any damage dealing spell with the 'good' descriptor that can handle that?

A:

This fine answer's suggestion of using the Advanced Spell Search tool to find arcane spells with the descriptor good is an excellent one. Pick from the results a spell that deals damage and smack some evil with it!
However, if a caster that wants to add the good descriptor to spells that normally don't have the good descriptor so that the spell's damage inhibits for 1 round the regeneration of a creature that has regeneration that's inhibited by good spells (like the horned devil), there is the metamagic feat Blissful Spell. The feat's benefit can be applied so that the slot the spell occupies or expends is 1 level higher and the "blissful spell gains the good descriptor." This should be enough to turn off for the next round the regeneration of such problematic creatures without resorting to pesky summoning.
Bear in mind, though, that the benefit of the metamagic feat Blissful Spell can only be applied to a spell "that targets a single creature." (See also Aiming a Spell for more on the differences between spells' various Target entries.)
This makes the feat probably okay to use with, for example, the spell magic missile… as long as the caster's only targeting one creature with the spell, the magic missile spell's Target entry reading Targets up to five creatures, no two of which can be more than 15 ft. apart. Yet this targeting limitation makes the feat incompatible with, for example, the spell fireball, as that spell—an area spell—simply has no Target entry. Worse, spells that create a ray effect like the spell disintegrate and scorching ray aren't, in fact, targeted spells; Aiming a Spell on Rays makes this clear, saying, "You don’t have to see the creature you’re trying to hit [with a ray spell], as you do with a targeted spell." This makes ray spells also incompatible with the feat Blissful Spell.
In other words, while the feat Blissful Spell is broadly applicable in the abstract, for this particular use the caster must pick carefully which spells to apply it to.

A:

Against Evil creatures a purely Arcane caster has 2 options
I used this advanced spell searching tool to look for wizard/sorcerer/bard spells with the [Good] descriptor. I got 16 results, mostly from the Champions of Purity splat book. Of these, 2 allow a Wizard or Sorcerer to bypass/disable Regen X/Good.
Burst of Radiance

This spell fills the area with a brilliant flash of shimmering light. Creatures in the area are blinded for 1d4 rounds, or dazzled for 1d4 rounds if they succeed at a Reflex save. Evil creatures in the area of the burst take 1d4 points of damage per caster level (max 5d4), whether they succeed at the Reflex save or not.

This is a level 2 Sorcerer/Wizard spell. Since it has the [Good] descriptor, the damage it deals will bypass Regen X/Good. The targets need to be evil, but that is what you asked for anyway.
Angelic Aspect

This spell functions like lesser angelic aspect, except you gain low-light vision, darkvision 60, resistance to acid and cold 10, and DR 5/evil, and you sprout white feathered wings allowing you to fly at a speed of 30 feet with average maneuverability. In addition, your natural weapons and any weapons you wield are considered good-aligned for the purpose of overcoming damage reduction.

This level 5 Sorcerer/Wizard spell is your only general-purpose option (if you find a non-evil creature with Regen X/Good). You'll need to physically hit the target with your weak Wizard arms, but the resistances, DR, and flying speed should work to keep you alive.
Oddly, Angelic Aspect, Greater improves on Angelic Aspect in everyway except it doesn't include aligning your weapons to Good.
As always when using material from a non-Core book you should get GM approval, but these are from an official source so you shouldn't have too much difficulty with that.

