Tyrzea it's definitely not worth it to equip the PLD soul crystal until after you learn Shield Oath. Because it halves the number of cross-class ability slots you get and you gain nothing of value until then.

As a note it seems like your singular problem is figuring out how to deal with multiple enemies. Easy hints:
Don't forget to use Flash with some frequency, add as it may seem as long as your healer is not going Medica-crazy Flash should be able to keep the hate of the other mobs.
Cycle through targets, deliver a few of your enmity+ attacks on the secondary targets to gain a bit more enmity on them.
Don't be afraid to let go of your current target and switch to a different one once the one you're hitting is at 25% health or somesuch, the DDs will finish it off before you lose hate and you can start getting a headstart on hate on the other enemies