The Sun domain already causes undead to lose their channel resistance vs positive energy of the cleric with the domain. What the spell does is to give that benefit to characters channeling positive energy without the domain and to impose an additional -2 penalty on the save. But the undead affected by this spell get a will save (their strongest save) to avoid those effects of the spell. This spell has minimal benefits to channeling positive energy to a cleric with the Sun Domain.
The real benefit of the spell is that it reveals undead. Many undead have good stealth rolls or can become invisible. The main purpose of the spell is to create an area that other people besides the cleric can defend against undead. As a way to weaken undead this spell is actually really weak especially for a cleric with the Sun Domain.
The spell you want to use to weaken undead is the 2nd level spell Consecrate. It increases the DC to resist channel positive energy by +3, and imposes a -1 on all attack rolls, damage rolls, and saving throw by undead. Since it is increasing the DC of channel energy and reducing the save the net result is the undead have a -4 penalty on their chance of resisting channel energy. It also has twice the area and lasts for 2 hours per level vs 1 minute per level and has a casting time of 1 a standard action.
Why would any cleric use a third level spell to weaken undead when a second level spell does a much better job?
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I wonder if this spell was originally intended to function more like invisibility purge? I still don't personally think it would be a good value proposition but it would be a lot easier to visualize a use case if the radius moved with you.
Or if it lasted 1 hour/level, you could use it in a "The undead are trying to sneak up on us, we just need to stay in this room until dawn!" haunted house scenario that wasn't full-time combat.
If you are going to be using Consecrate invest in a weapon with the Reliquary property. It lets the weapon function as a holy symbol and counts as a permanent fixture of your faith while it is in the area of Consecrate. It only adds 250 gold to the cost of the weapon.
Even if you are not using Consecrate it is a good investment for any adventuring divine spell caster.
For an undead focused cleric make sure to also take the glory domain in addition to the sun domain. The extra +2 on the DC makes it even harder for you channel energy to be resisted.