Ok, I found my way.
I very very rarely had used potions before so I have simply a stock of potions stacked up almost from the start of the game.
Team drink speed potions, Jennell summons a drake which keeps Zephyrine at bay as we hammer down the awakened watcher twice each round. The highest average damage is inflicted by the shaman, btw.
I tried with Nathalie to no avail. As she was the one since I've known her, Jennell saves the day yet again.
As soon as I take down a guard, I run away, leave the region, let the team rest to full HP and prepare the skills ready to go, I return to ram some more guardians -this will prevent at least 3 potions of each kind, I predict.