I'm not sure what encounters you are talking about there are random encounters you get by touching the blue or orange flames and story bosses encounters you get at the end of a Level.
So i don't know if you can encounter her once again, but here is some advice on how to get better at fighting in this game.
Firstly grind the area around Camp to get some better gear and Level up.
Secondly there are some red and blue orbs that randomly appear around the floating islands while exploring , red ones increase your HP by 10 and the blue your MP by 1 .
Thirdly there are these grey statues around the map, they restore your HP and MP use them often to keep your HP and MP.
A little extra tipp use the heavy blow ability to deal a lot of damage to her. You can use the ability from the start of the game, but game never tells you that you have it. Or at least i overlooked it at the start.