Bellatrix means battle maiden and Ottilie means prosperous in battle! I love both though Bellatrix is a brave choice in both senses :) I also adore Matilda (I have one!!!!!) and Althea... Another favourite! My daughters middle name is Valentine...another option that means strong! It appears I'm a fan of strong names for girls too!! :) good luck!
Brunhilde or Boadicea! (Alright, alright... probably not to anyone's liking....)
I think it's difficult to nail down a strong name exactly. I tend to think of German names... The first I thought of was Millicent (and then I looked it up and it means strong!) but I like the original form Melisende even better. Other names that I though of: Bryony, Gretchen, Deborah, Joan, Louise, Marcheline, Rosamund.
I tend to think that shorter names sound stronger: Scout, Beth, Ruth, Eve, Leigh, Anya. Two names which are now my favourite girls names (ever, probably) which I think are very strong and pretty but not frilly: Roberta and Erika - so much love! Also; Athena and Minerva (great twin names in my opinion). And then some random last minute suggestions which I've just thought of: Imogen, Heidi, Freya, and Phoebe. :)