You can disable the boulder first, go back to a town (shouldn't be tough with the EC), and buy a level 2 EX gem for 100 grade. That might be your best choice right now.
as for Efreet and Sylphs: 3 melee characters + Raine all set to "scatter". Trust me, I took them (them I mean Sylphs) out with +20 grade. btw, try to time your attacks so they chain with the end of your allies' attacks so efreet stays staggered (or be cheap and just go a, a, *angel feathers*, defend to cancel angel feathers, repeat to go for infinite combo...but I have to tell you that it's difficult to pull off without hurting yourself seriously first...and he needs to be in a corner...well actually that's how you beat nearly all of the 4-on-1 matches for the +10/+20 grade ratings).
As for the Sylphs, it shouldn't be a problem if Raine maddingly casts nurse while the rest of hte characters have 1-on-1s with each sylph.