To get to max xp you will have to complete quests off of the quest board. There is 1 in each of the plant and zombie bases. These look like a big notice board. Press b to view the board then there are 3 tabs zombie, plants and multiplayer. To select one of these hover over the desired quest and press a. To delete the quest if you have chosen a wrong one press y.
Every day there will be new challenges added to help you if any are difficult or time consuming. These range from such tasks as kill x amount with... To capture x amount or deny 10 orbs etc etc
There are 4 increments from the base of 1.0. 1.25, 1.5, 1.75 and 2.0. I have t got as far as 2.0 yet to confirm if this is max do or not yet so will need confirmation.