So, I finally reached this insanely overpowered lich... and promptly got my rear handed to me! Which was really a little strange considering that, at level 62, nothing had presented me with such a challenge in quite some time. But I was persistent, and really determined to defeat her. So, I loaded up on resistance potions and scrolls, grabbed my best enchanted spear, and tried again. This is where things became even stranger...
With the resistances I had, I managed to stay alive for more than 30 seconds, so that was a step in the right direction. But no matter what I did, I just couldn't hurt her... I mean not even a little! I was getting pretty frustrated... frustrated enough to leave the game and do some research...
Well, my research ended on the "discussion" tab of Gedna's page on the UESP Wiki: http://www.uesp.net/wiki/Tribunal_talk:Gedna_Relvel#.22Leveling.22_Bug. As you can see, there is a problem with the script. Instead of calling ModHealth, ModFatigue, and ModMagicka, ModHealth is called three times! Also, as the author of the entry points out, the MPP only fixed the part of the script that gets the players level, effectively making matters even worse: Unpatched, I believe her Health would have been set at 700 no matter what your level, because "GetStat Level" would not have returned anything. I'm not exactly sure how Math works in Morrowind scripts, but assuming it didn't just crash altogether I imagine "GetStat Level" would have assigned either 0 or NULL to the "level" variable, so the calculation should have defaulted to 700 HP regardless of your level (I assume this is why you occasionally see a post from someone saying "What's the big deal? I had no problem killing her!")
Soooo, at level 62, my poor assassin was facing a lich with incredible resistances to almost everything, and a whopping 50,300 HP! Going by the way the script should work, she should have had more like 6,900 HP, which is still a lot, but at least it's not an impossibility. Using the console I set her Health to 6900 and tried again. It was still an epic battle... I went through all of my resistance potions and scrolls, and in the end I resorted to Hand-to-Hand combat (luckily, H2H is a skill my assassin had developed... otherwise I might still be fighting her!) (Also, running BTBGI definitely was not helpful in this case! )
Anyway, I was a little surprised to find that there isn't much about this scripting error here on the forums. I guess I assumed something like this would be more widely known. And it does seem that at some point someone tried to address Gedna in one of the MPP's iterations... they just missed the bit that really makes a difference (and like I said, I imagine they made her even worse by correctly getting the player's current level!) So, I started this topic. Maybe it will be helpful to someone who, like me, makes the mistake of taking on Gedna Relvel at a relatively high level. Doing so definitely is not advisable!
EDIT: I forgot to mention... Yes, I did indeed open up the CS to confirm the MPP's version of the script. It is exactly as shown on the Wiki discussion page I linked to. I meant to mention that, but that damnable lich has me all discombobulated!