I am not so sure about that, I have been tossing around build ideas for Phantom and everytime I would be better off by using Rogue. Whether it is feinting or sneaking, the Rogue gives the best results. The only two build concepts where I would suggest Phantom over Rogue are as follows:
- Wisdom based Phantom 3/Cleric 7/Darkfire Disciple 10/Hierophant 10,
- Phantom 4/Wizard 6/Arcane Trickster 10/Something Else 10.
The first build benefits by getting wisdom modifier as AC and having three uses of instant invisibility spell that lasts three rounds.
The second build can also benefit by having few extra points of AC from wisdom modifier, and having four uses of instant invisibility spell that lasts three rounds.
But the fact remains that Rogue is a favored class for Svirfneblin, Halflings, and Tieflings while Phantom is a favored class to no race. If you wish to avoid the crippling multiclassing experience penalty, the list of viable races to make use of Phantom is shorter than with Rogue. Not to mention that Rogue levels open up a far more expansive list of skills in addition to granting both Evasion and Trapfinding with just two levels.
The above two builds will not be able to disarm traps with higher DC than 20, and have no access to Expose Weakness unless the cleric build goes for the Water Domain.
The above two builds are the only ones where you can actually have a proper argument over whether you should pick Rogue or Phantom.
***
Hence my suggestion to grant Spell Resistance to Phantom. But I admit that permanent spell resistance might be 'OP' of sorts, especially if you consider how Phantom would have more feats to spend on Epic Spell Resistance.
So how about a
Ki Spell Resistance?
You use X amount of
Ki Power to cast the spell '
Spell Resistance' where your caster level is your level in Phantom class. Therefore, if you multiclass, you will have less spell resistance and it is more likely to be dispelled.
The shared flaw of both Rogue and Phantom is their horrible Fortitude and Will saves. With Ki Spell Resistance the Phantom could off-set that flaw to some extent.