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If you kill all of the Psychos and complete all of the side missions, you should be able to reach level 50 by the end of your first play through. Once you unlock the more powerful combo vehicles and super combo weapons, leveling will come exceedingly easy. The biggest tip to leveling is to create massive combos. To further boost your combo score, you should consistently use your special execution moves. Also, witnessing Tragic End events and collecting the Limited Frank Statues will net you some PP. If you are having any issue leveling, refer to some of the other guides here on Steam that provide an actual walkthrough of Dead Rising 3.