You reported yourself that +3 weapons *already* hurt my version of the GWW. Or did I misunderstand?
Ups, it was a misunderstanding here, Littiz, sorry for that. All started with Maestro asking how the usual GWW should be (and it was my fault, because I deviated the debate a bit, I apologize for that) : my replies were for this question.
But, until now, I replied relying only upon the info from forums and the mod's docs, which was not quite fair from me. So I fully tested your GWW form. What were the conclusions :
1. First test : two companions attacked the GWW with Gram sword of the Grief and the Ravager. It was as if they hit him with sticks.
2. Second test : GWW against Improved Firkraag (and the GWW had no support from other party members). GWW destroyed Firkraag without taking even a scratch. Firkraag's claws were useless against GWw. Firkraag was able to damage him a little with his fire breath, but GWW regeneration rate was to fast for him.
3. Third test : GWW (alone) against Improved Abazigal. GWW destroyed the human form of Abazigal (who was enable to hurt him with his +5 sword). The dragon form of Abazigal damaged him a little with electrical attacks, but again GWW regenerated too fast. But, also, GWW was not able to damage too much the dragon form of Abazigal. So, they harassed each other almost 20 minutes, until I got bored and I gave up.
4. Fourth test : GWW against Improved Demogorgon. Indeed, the Demogorgon was the only one able to kill the GWW, but with a wave of spells, not with physical attacks.
Yet, I don't know how the GWW would do against enemies with draining attacks (like vampires or illithids). I forgot to test that too, but I think it's enough.
So, the weapons of all those bosses were completely ineffective against GWW.
Now, let's analyze what options you have :
1. Change some weapons to be cold-iron or weapons. In my opinion, it's not a good idea. It does not make sense to create a immunity, and after that you change the weapons to pierce it.
2. Make GWW immune only to +2 weapons. That would be a better solution, but it has a disadvantage : you did a nice work with this immunity, and this alteration would ruin it.
3. A third solution (which I think it's the best) : edit the GWW shapechange so it would last only 30 seconds (or 40, or more, or how long you think it's better). The main problem was that the Shapeshifter could remain in GWW form a VERY long time (by the way, how long this shapechange lasts ?). This solution has the advantage that the GWW would remain as it is, you would preserve your work, but the game won't be so unbalanced.
4. The last option : let the mod as it is. You have this option too, but remember that the GWW is more powerful even than the Raveger itself.
You choose what option you wish, because, even if I suggested the third one, I don't know how much work it involves. And, by the way, the test were made without the ability Feral Spirit.