Upon chasing down Sylphie you'll fight a battle against a Tainted Earth Sprite that summons waves of monsters to defend itself. Focus on taking down the additional monsters. While the NPCs are capable of healing each other, you should step in as well if you want them to last the whole fight.
Kill the initial wave of Mudpie enemies as fast as you can, Then focus on the getting the Treants down as fast as you can. If you do this the NPCs should manage to make it the whole fight. they may still need some minor healing though so keep that in mind
Eventually, elementals will appear to assist you. The waves of enemies are not limitless, and once they are exhausted you can focus on the earth sprite.
Another strategy is to focus primarily on healing the NPCs. They will eventually take down the mobs along with the elementals. The NPCs do not need to complete the duty.