This should be a fairly easy trophy to get. Level 14 is the max for level for your Guardian each match (your Guardian level is not persistent, it resets each match). Just focus on killing the creeps (the soldiers in each lane) and other guardians and you should hit level 14 by the end of the 20 minute match. Avoid dying (seems obvious) so you can spend as much time in your lane as possible to get max XP. If you are having issues I guess you could pop an XP Potion or two, but I would not recommend this...hitting max level is not terribly difficult.