in the caves you should see three volcanic vats in the ground,go to the vat that is
pouring smoke when the serpent is about to attack shot your flame thrower into the vat
which will cause it to explode and hit the serpent as its coming down to get you. Note:
once youve hit the first vat the position of the smoke will move to the next one so you
have to keep moving back and fourth. Also if youve got napalm grenades in your flame
thrower shot them at the serpent aswell as it damages it.Hope these hints help.Dont worry
it took me a while to defeat it aswell good luck. =)

wen i kill the serpent i first go into the cave and start shooting the serpent and lewer
it closer to me then i jump to the other side of the serpent and start shooting it from
behind .then i shoot the vats.this way it makes it more exciting.