If you're really stuck on the Root Core, try these:
-It's weak to both Fire and Ice
-It always uses Magic Pulse on the first turn, which is a strong attack that also recovers your MP. Guard it.
- Magus and Marksman have the easiest time on this boss. If you're playing as a Myrmidon, though, be sure to keep your taunt up so the rest of your team can do more damage.