Go to the Backyard Battleground
Go to the Quest Board
Complete the quests to earn a recommended 15 stars for other trophies (though you only need 5 for this trophy)
Go to the Flag of Power and (from the Plants perspective) go right, drop off the ledge and the sewers will be behind you.
At the end of the sewers in the big open room, go to the back and lowest level.
Here you will find a door to the Crazy Target Range you will need to open
I used the peashooter personally but any character should do