You have to reach level 130 and then purchase a tier 6 item for 1500 coins. Play survival mode for maximum exp gain. Make sure at least one person chooses Jupiter for their patron god. The best combination is one player as Jupiter and one as Apollo for focus and health gain. Always strike your enemies enough to make them executable and then perform the executions. After the first two, you should have enough focus to begin using your special, which should be slow time for both players, stunning your enemies and allowing you to get an additional 4-6 hits on your enemies before executing them. The goal with all of this is to maximize your combo. Whenever the skull on your screen starts flashing, you are about to lose your combo. Press LB to boast before the crowd, which saves your combo for about fifteen seconds or so. The higher your combo, the more experience you will gain in the round. It follows an exponential growth rate, so very high combos result in massive experience. I gain three levels per survival match with each match taking about eight to ten minutes. A good team can go even longer.
EDIT: corrected the name of the gods