The first time you run around avoiding his attacks til he does a spin
drive into the ground then hit him with any weapon. he will shed blood
so you power up with the power of Hercules and hit him with it 2 times
and the ceiling will fall and he will disappear
the second time you will see 3 red pillars in the arena. you dodge
phoboses attacks till you hit him enough so you get the power of
Hercules to break the pillars. once you've done this hit phobos into it
the light 2 times. If the light wears out break another one.

P.S. phobos will shout out a green slime after he is hit into the light.
it breaks up into little pieces if you get a distance so get a distance
and dodge that attack because it does a lot of damage.