http://nwn2db.com/build/?262307
This is what Phantom 21/Assassin 9 would look like.
vs Phantom 25/Shadowdancer 5:
- You lose Epic Dodge.
+ You gain 3d6 Sneak Attack.
+ You gain UMD as a class skills (can use scrolls).
+ You gain spells (Cat's Grace, Fox's Cunning, Freedom of Movement, and Clairvoy/audience being among the useful ones).
+ You gain Death Attack (Paralyze upon Sneak Attack when they fail).
+ You gain Uncanny Dodge.
+ You gain 2 feat slots (no need for dodge/mobility), which I used for Skill Foci in Hide and Move Silent (didn't really need much else, and HiPs is your primary defense. Pays to maximize it.). You also gain an epic feat slot, which you would have overwise used on Epic Dodge.
You do also lose the shadowdancer's shadow daze, summon shadow, and shadow evade. You also gain resistances against poison and improved uncanny dodge, but all of those abilities are fairly useless so I didn't include them in this list.