[Olorin]

If you need to take out a mage with stoneskin, there is still a fairly easy way. The key is that the poison weapon skill applies to ranged attacks as well. I gave my assasin the Tuigan bow (for the ApR), and kept some arrows of dispelling handy. The first hit with the arrows will dispel the stoneskin, and subsequent hits will poison the target.

I just tested it by having one of my mages cast stoneskin, and then attacking with my level 22 assasin. I fired one arrow, and it took out the stoneskin but did not poison. I fired a second arrow and it poisoned the mage. I let the poison run its course and it did 30 points of poison damage. Furthur testing against fighter types and the mage again showed that if the victim makes a save vs. spell, then they are only poisoned for a total of 6 damage in about 2 rounds.

The thing to remember is that poisoned effects stack (both the assasin's skill--you can use multiple poison weapon skills at the same time--and each hit injects another dose of venom into the target). If you can hit a mage 3 or 4 times with poison, they will take that many times the damage.

If you are going to use the arrows of dispelling strategy, here are a few useful tips that I've learned:
1) Once you have dispelled a mage's protections, switch to other arrows--arrows of dispelling are fairly rare, and they don't do much damage
2) The mage spells protection from magical weapons, mantle, improved mantle and absolute immunity will make the arrows of dispelling useless (arrows are +2 enchantment, do not "hit" under these spells, so the dispel effect is not applied--same principle applies vs. enemies that are naturally immune to +2 and lower)
3) Find ways to maximize your damage. I like the Tuigan bow for it's extra attack, which is more opportunity to hit them with poison. I also like to shoot poisoned arrows of piercing, or acid, or better yet biting to get the most damage possible.
4) If you're after a tough mage using the dispelling arrows, don't have the rest of your party attack them right away. Since a lot of mages have contingencies set up to refresh stoneskin or cast prot from magical weapons, you want to make sure that if you're only going to get one unprotected hit against them, it's a poisoned one.

If you have installed the improved mages mod but don't want your assasin to be relegated to a ranged attacker, consider starting combat with a backstab against an enemy cleric, and then shooting at the mage. This way you will poison two of the enemies casters right off the bat. Since enemy clerics tend to cast for a round or three before going to melee, it should be safe to fire at the mage while standing next to the cleric...just run for it before the melee fighters can get to you.