There will be these blue and red "flowers" that spawns on the ground, 3 at a time. So 3 blue and 3 red. What you want to do is grab 1 (assuming you're doing 6-man) flower of the opposite color of what snake lady you're assigned to.
So say you're attacking blue snake lady, then you need to grab 1 red flower when it spawns. After about ~10sec of flowers spawning, any flowers remaining on the ground will be eaten by the boss and they gain health from it. Having the opposite color of flower from the snake lady you're attacking is a must for this encounter or else you'll die.