Disclaimer: there's currently a bug with the pommel in which it will not recharge after using it. If this happens unfortunately, there's no way of preventing or resolving this that I am aware of. You would just have to restart the EE up until you receive the pommel again.
You receive the pommel after doing all 3 rituals and filling the pool in the ritual room with enough blood. You will see a part of the statue light up, go up to it and press X, a notifications will pop up.
Now that you have the pommel I advise that you finish the Easter egg if you have to. You do not want to do the boss fight on higher rounds. Holding LB charges the pommel. After using it, the pommel takes a few seconds to recharge so don't worry about running out of it.
After finishing the Easter egg you can do this in 3 places ideally(excluding glitches). Do not do this by the pack a punch machine. It's a waste of time and you'll end up buying shield and wasting kills by shooting zombies while waiting for your pommel to recharge. Same goes for ritual room, muechlers will spawn there and charge at you, DO NOT TRY IT IN RITUAL ROOM. Speed perk is advised as well for this.
Location #1 Up by the bunker, this area has a lot of space to run around and train from the radio to the artillery into then bunker then around in a circle. Let the undead spawn in, charge pommel by holding left bumper and throw at your feet. If you get low on shield you can purchase it in the bunker.
Location #2 Beach. Tons of space here to train and run around and in between rounds you can head up not to far and buy more shield if you need it.
Location#3 U-boat pens area(where you place the decapitated head)
Run around in circles training zombies, charge pommel, throw at your feet. Rinse and repeat. Shield is just up the stairs if you need it.
This took me 5-7 rounds after Easter egg, not including boss fight kills. Good luck!