believe me, that monster gave me more than my fair share of frustration. (especially trying to make the solomon's ring for doom train.)
if youre too afraid to fight it, (which is perfectly understandable), equip "initiative" and "mug" to a character, and just try and steal the tentacles. if you fail, run away, and try again. (you can run away when statused.)
at least, thats all i saw the point of fighting marlboros. or try junctioning all of those spells. (but good luck getting them all...)