Thymesia

At the end of the Royal Garden level, you'll find a boss.

This boss is not that hard, I beat her on a third try. The best way to be victorious for me was: Use the greatsword plague weapon, this sort of stuns the beast a second or two, which you can follow up with a Reave claw ability (to steal her ability) and double dodge a lot. Every time the creature jumped up to slap you, I double dodged around it, and use your saber and/or claw for some damage. Repeat!

Dodge is also easy on her spitball-wave attack.

When she jumps up and curls up, she'll do a big attack. Either tank it or run away.

Also, when you reave her, you'll leech a health regenerate ability. Try to use this instead of your flasks, and you should be having enough healing power left over should you need it later in the fight.

Take your time, don't rush it.