SWORD OF LIFE's guide is good but incase you are wondering there are a couple relics you can use to help you get some additional experience with every kill. It isn't much but it helps, unfortunately there are only two. To get Unholy experience in case you don't know, you have to punish them. (Usually X when given the option)
CAUTION, SOME SPOILERS AHEAD ON ENEMIES YOU HAVE TO KILL
The first you find is Tail of Minos in which you get for defeating Minos. Punishing anyone you get additional Unholy experience.
The second is Francesca's Book and it's found just after you defeat Minos where you swing on some ropes you'll see a pair of vines, when you swing to them climb up to reach the relic. Every 50 kills with the Scythe you gain additional Unholy experience.
This will most likely take you two playthroughs to get both Holy and Unholy (one for each playthrough), so go for one first and when that's finished go for the other. Punish every little thing you can as long as it won't get you killed and good luck.