Due to the insane quality of the upper-tier combo weapons, leveling in this game can be incredibly easy once you unlock them, and the more powerful combo vehicles. The most important rule, when leveling, is to combo. Massive combos are more easily attainable by the super combo weapons (which is why they're awesome), and increasing their durability through books and skills can allow you to carry combos for longer periods of time. To capitalize on your combos you should also use special execution moves to get a boosted score for a single kill. Your list of execution can be found in the player menu of the game, with the initial one being the basic or weapon specific execution. All of the executions can be conducted when the prompt pops up on the left side of the screen, next to the combo, and they should be used on the tougher enemies to make sure they don't get in your way during the combo, and also because they give the most PP and will utilize the current multiplier the best (these include King Zombies, Prisoner Zombies, Football Zombies, etc.). When going on PP gaining runs, try to keep as many combo weapons on you as possible (use the list below for the best weapons and locations, rated by their power level).
Note: The locations aren't static, meaning if you were to say pickup the blueprint listed for Ultimate Mecha Dragon before any of the other Mecha Dragon blueprints, you will get the Dragon Blade blueprint first. You would need to collect the other three Mecha blueprints in any order to find the Ultimate Mecha Dragon as the last one.
-Dragon Blade - House NW corner of map
--Flame Mecha Dragon - In tunnel near flame trial in South Ingleton
--Mecha Dragon Blade - On top of shack in alley NE of Rhonda's Garage
---Ultimate Mecha Dragon - Roof across from garage. Enter rear of Northern building through back alley.
-Reaper - Alley West of garage
--Fire Reaper - In parking lot/courtyard of apartment building NE of Rhonda's Garage
--Death Mask Reaper - Melee weapon shop North of garage
---Ultimate Grim Reaper - On top of building next to small park in Ingleton
-Electric Staff - Back alley in SW Almuda
--Electrofire Staff - Fire station in N Almuda
--Electroice Staff - Roof of Almuda Farms, across from bus depot
---Elemental Staff - On top of power grid area in Almuda
-Power Shout - Roof next to Almuda garage
--Electric Shout - Roof of building across from Fire Station in N Almuda
--Super Shout - Roof of Almuda auto body shop that has a trial and tragic end on it
---Ultimate Shout - Dumpster next to construction site in Almuda