CONTAINS SPOILERS
Trick to defeat Rykard & God-Devouring Serpent
Once you enter the boss area, immediately to your left will be a glowing item, the Serpent-Hunter. To defeat the God-Devouring Serpent (and later Rykard, Lord of Blasphemy) you'll need to equip this weapon. You'll want to have adequate FP in order to use its skill Great-Serpent Hunt which is a long range attack that can hurt the boss from a distance. Spam the skill and attack relentlessly.