Fort's a rough one if you've murdered a huge boatload of good-aligned characters and don't have much in the way of DR built into your character. I typically lack the attention span to hold onto a character that long. Ktaar probably would've had an easier time as a new hero, oddly.

Beyond that... I've played a number of similarly built characters that didn't struggle with guardians as much as you seemed to, so I'm curious:

1) What kind of strategy did you take in terms of how you were geared? What about weapons? For a felar assassin, I'd fight an awful lot of PKs with a one-handed sword, but I might well switch up to staff or spear to fight an inner unopposed, wanting that extra tanking over the backfist damage. Even though the tanking isn't exactly the problem, any damage you save is less damage bind wounds needs to keep fixing and more of the direct damage it can eat into. I'd have to try it both ways again to see if that still holds up.

2) What about to actually fight the guardians?

3) Would using some non-dispellable DR have been a workable option? For example, let's say dispellable protection is easy for you to get. Might it have been worth your time to use it and reapply it after each successful dispel?