Liz has been excellent throughout the series...but on the two occasions in the boardroom when she has needed to justify or explain her performance she hasn't been as strong as she needs to be to win the competition.
I am thinking initially of her week as PM...she did ok, but not as well as others (eg Stella) and in the boardroom she was a little bit on the back foot. I suspect she wasn't under that much threat tonight, but she was slightly flustered when asked to explain why she shoudn't be fired, and otherwise she was either edited out or simply not saying that much whilst the other two were fighting their corner. Sugar is contrary enough to fire her just for saying she was surprised to even be there.
Liz has generally been very accomplished in the tasks , but will need to be stronger under fire by the time it gets towards the final episodes. None of the other candidates has really had a go at her yet, but if we end up with Liz, Stella, and Joanne (say) as the last three then she'll need to be able to hold her own in an argument of equals and we haven't seen it yet.