Ok, the simple way, even if you have zero preparation for this method will get you through this fight with minimal headaches and time.
Invest in Divine armor. Your magic can't really do the trick here so best to use it to heal yourself. Also will prevent you from taking as much damage. Useful in both stages.
Beef up your cross. In stage one it is your best bet. There are relics you can use to make it stronger and I highly suggest you use them.
Basically you just play dodgeball with the devil until he gives you an opening. At first it will appear as a button prompt in the circle of light in the center. You will pull yourself up to his eyes w/ your scythe and attack him (I suggest with the Diabolic Rupture Y+Y) to make his reveal his vulnerable stomach. Hit him with cross attacks. If you can activate redemption before you zip up to him. makes it that much faster. Eventually you wont be getting the prompt which means when he puts his face up there like you should be just hit him with the cross until he goes belly up and take him down. Use Divine armor when needed to heal.
Easy way is to invest some points into an unholy relic or two that improves scythe damage and use the Soul Stabber LT+A combo (level 1 unholy so not much effort to get) and just spam him with it. It blocks most of his attacks because it hits him enough times to make him stumble. Better than cross attacks because A. he can't counter attack and B. he can't get off those dangerous spells he hits you with. Once he starts flying just use the prompt to zip up to him and use quick scythe attacks to wear him down.
Done in a few minutes and most of the moves used would be bought to use in the main game and the ones that might not be are cheap.