This is the way I use for grinding out levels on a character.
Guide to Earn Lots of XP Solo
You need to have or earn 3 stars for the decoration in the zombie or plant base.
You need to go up to the decoration.
You need to press the edit decoration button and the pause button at the same time.
How do you know it worked?
When you press both those buttons at once your camera should be in a drone like view while your pause menu is open.
Go into the opposing base to whatever team you are and trigger the alarm and start fighting the bosses with your invincibility.
It majorly helps you if you have your xp gain boosted by 2x via board quests.