I was killing hr and got the message that I cant soul trap her because I dont have a large enough gem. I have Azura and a grand gem. Do I need a black soul gem to nail her or she just wont go. I also have that colossal one with the Archmage but I am guessing thats for the main quest or whatever.

Or is it bull that she can actually be soultapped?

Do I need to kill her with a soul trap spell like any other creature or just any weapon?

I think I read you cannot soul trap people but she looks like an exception.

To capture any NPC's soul you need a black soul gem. Umbra's soul is the same as any other NPC.

You can sometimes find Black Soul Gems on higher level Necromancers. Or you can make them your self (you find that an dhow out in the Mages Guild questline)

An NPC (Non Player Character) is roughly any character that can talk. So People, Elves. Dremora Lords, Dark Seducers, Golden Saints are examples of NPCs and require a Black Soul gems.