Every time you level up one of your Zombie or Plant character (by completing Challenges) you also gain one Player Level. To unlock this trophy you will need to gain a combination of 50 character levels across your Plants and/or Zombies.
In the process of getting the Crazy Dave Squad, Zomboss Recruit and Challenge Accepted Trophies you will have accumulated a total of 37 Player levels. You'll just need to grind out those last 13 levels by completing Challenges on your Plant and/or Zombie characters. Pick two or three that are enjoy playing and work on those challenges or you can save up your Challenge Star Skip consumables for this last push to level 50.