1.) You will need to complete each God's challenger by interacting with the gong at the top of each tower.
2.) Interacting with the gong will spawn in either 1-3 gladiator zomies or 4 zombies tigers.
Once killed, a severed challenger head spawn will be available for you to collect.
3.) Repeat steps 1 and 2 in the other 3 towers
4.) Navigate to the temple (bottom of the map) and place the heads on the 4 pikes. After a few moments PaP will be available.