I climbed it in the early '90s so my recollection is hazy, but I can say that the description in the AC guide gives (or gave) an accurate picture.
Technically, the hardest pitch would be the Fissure Allain. It was fully pegged up when we were there, but *looked* like about UK 5c/6a to me if done free. A hard Millstone crack, anyway. But that is now generally avoided, as we avoided it.
As for the rest, I remember it probably didn't get above UK 4c/5a. But watch out for verglas which can make the grade irrelevant.
The Fissure Lambert was a thrutch. My lead, and I remember hand jams in the bowels of the crack between rock and ice!
A very satisfying route though, and more-or-less objectively safe once you're past the niche.
In reply to abcdefg: free the hardest.bit is prob (we frigged it) e2 , we found a fair bit of hvs ish ground. Took a while to get on the face and we climbed quite a.bit of choss. The descent is complex. We were lucky and met a team descending what they had just climbed. It's a very satisfying I've just climbed that sort of feel but the climbing in summer is only good in patches