I’m new to this, so take my advice with a grain of salt – It reads a little confusingly right now – maybe re-arrange the order of some of the elements –

“Thinking she’s the last of her race, a lonely barmaid must fight for her survival after discovering the power that sustains all life while being hunted down by a vengeful superhuman.”

I think “depends on her own survival” & “fight for her life” are redundant, just pick one.
Try not to use the same word twice – “life” I replaced one with “survival”

I hope this helps.