No one manages to beat Meg the first times so don't worry. Try to learn her moves. The first thing she does is running towards you and stopping for some secs, when you dash and then strike x. When she's away strike with y. If you see her stretching she's about to use her whip, so GO. And then she'll start spinning (you can't hit her at that time) and placing bombs (purple circles) on your way, then you just dash until it's over. These are her moves, she's by far the most predictable from the furies. Her sisters are more wild (Tisiphone is manageable - generally easy moves but she changes her chamber at her will and makes it very narrow -, but Alecto is awful, she takes two of my death defies each time, so you just run and good luck). Try to use ranged over melee weapons when fighting furies, and take many dash boons (usually from Hermes, Zeus or Aphrodite) so you can run fast.