So I just arrived at the Fortress of Regret, and I'm pretty much getting raped by the true shadows. My TNO is a level 13 mage with some good spells, but nowhere near enough to kill the dozen or so shadows I run into in the very first room of the fortress. I'm thinking of coming back later as a high level fighter, but I don't know if it's possible to "grind" for XP in this game since I've already done pretty much every quest I've been given. Is running away an acceptable behavior at this point in the game, or am I going to have trouble with the bosses later on anyway?

Let me tell ya, I ran my ass off :).

I entered the fortress 'just to look around a bit', planning to leave or reload so I could go shop for supplies and such. I was really not well-equipped for long or numerous battles. Running from the shadows was really my only choice, and I ended up finishing the game without ever going back for the supplies I thought I needed. I'd say keep a save ready in case you feel really overmatched, but don't worry about fleeing.

As for your final boss question, it follows in the same vein as the rest of the game. The encounters vary based on your play. I'll say that there isn't much fighting after the shadows, but I'm hesitant to elaborate even with the spoiler-protected thread title :). Good luck though, you're in the home stretch!

There are many ways to grind XP in PST. You can kill the respawning thugs, black Abishai or other monsters on the streets, the monsters in Undersigil refresh every time you enter this area and if you buy the mini Modron from the curiosity shoppe you can use it to get to a training location.

Alright, managed to do it by running away from the shadows. I was lucky enough to still have the bronze sphere, gained about 8 levels talking to my past incarnations and killed the last boss spamming high level spells. Thanks for the answers.