You can level up a World of CHEL pro/player by playing in any of the four game modes [ProAm, ONES, Drop-in THREES, and EASHL], after selecting "WORLD OF CHEL" at the main menu.
PRO-AM - (300XP per 9 minute game/11,700XP in 6.5 hours)
I highly suggest going through this game mode before the other modes as I believe it will level you up the fast at the start and is the only mode in which you go against the AI not human players. Each game will take about 9 minutes and you will gain 300XP for winning/completing a game. You do not receive extra XP by scoring goals or other tasks, so the best way to go about this is score 1 goal and hold the puck behind your net for the remainder of the game or there is a nice cheese spot you can sit with the puck between your defensive blue offsides line and your zones faceoff red outside circle where the AI will glitch-out/not try to get the puck from you. I do believe the difficulty gets higher as you get further but its nothing too difficult and you shouldn't have a problem completing all 39 games, netting you a total 11,700XP.
EASHL or Drop-in THREES - (150-400XP per 15 minute game)
If you enjoy the NHL series and have played the previous games then you may want to play/spend most of your time in these game modes. These are very competitive modes that have you facing against other human players and will require at least 2 players per team or 4 total for the game to start and continue. However you will gain 100XP for completing a game, 100-150XP for winning, and extra 50-150 XP for personal tasks/performance you did in game. Each game can last from 12 to 20 minutes with setup and load times. Although if you're playing a game and the other team you are against quits, you can obtain this XP much faster but only for the winning team, the losing/quitting team will not gain XP.
ONES - (75XP per 5 minute game AFK/Idle)
Now if you don't enjoy the game or have no interest in the other modes or anything competitive or want to gain XP while not having to play [sleeping or at work]; I'm happy to say you can completely idle XP in this game mode. This game mode is a ONEvsONEvsONE human player game to see who can score the most goals against an AI goalie. Each game will take about 5 minutes with searching and load times and you will gain 75XP per completion/game. You can be completely AFK[AwayFromKontroller] as after the game finishes it will automatically search and put you in another game without input from you. Although I do not know if it is my internet or EA but seems as if after 20-30 games I disconnect from EA servers or come back to my screen sitting at the ONES main menu. Just beware.
TOTAL XP NEEDED TO HIT LEVEL 1 PRESTIGE 1 / UNLOCK ACHIEVEMENT = 44,750XP