Not proud of it, but just beat the second dungeon after using 3 gems. I have the required units (def break, heals, status clear/prevent, etc), but just didn't have the damage to pull it out against Juggernaught without letting him kill my guys and then gemming to get full SBBs to burst him down.
Comp was:
Lillith SBB 4 (lead)
Lunaris SBB 3
Zephyr (5* maxed)
Elimo SBB 5
Gravion SBB 4
Dia SBB 1 (friend lead)
And just beat the last dungeon on my second try, no gems this time. MUCH easier than the first.
Lilly Matah (lead)
Lunaris
Zephyr
Elimo
Lillith
Michele (friend lead)