A senior girl at work is making it difficult?

Well she sounds like an awkward person who could have been managed until you said it's making you look like you don't know what you're doing. If you feel this is so then you need to keep an eye on her and keep reporting the difficulties you're having to your boss. Say stuff about the issues you're having with time because she is making each task take longer to complete for some reason. This should get his or her attention while at the same time keeping you in the clear of any potential blame. She may just be being awkward but I've been there when some individuals have cultured a whole environment which has eventually lead to someone being marginalised, and it starts with the person being undermined in some way, either by this individual alone or in front of other team members you work with. So if this is her intention then you may find she'll probably do something like this but in any case keep your wits about you.

Just accept that's how she is and work round it. If you really know what you're doing, that shouldn't be too hard to do.