in a match you level up by taking bases and killing units (especially the main AIs)
you get more xp from the AI so i suggest keeping a tight hold of whatever bases you do have on the map, dont take any over yet and just keep killing the AI and their units for xp, once you're around mm lvl 12-13 start taking over bases and win, you will get to lvl 15 by the end of the match than.
Note: during the game you have xp boosters you can buy as you lvl up, i suggest getting them:
lvl1: +7% xp
lvl2: +14% xp