I saw 2 witches from corvid souls and an additional 2 I created from frags, it isnt that hard to get her at all. This is how an event should be run. The more you play, the more "extra" characters you will have to sacrifice and also, you will learn to store all souls until the sac event so you maximize the chances of getting the event souls and frags. It also helps to stock up on gold to be able to sacrifice. I blew about 18M gold (plus all the gold you earn during the sacs) from about 28M down to 10M. I am now already back up to 20M just in the last few days playing arena.