The Completionist Achievement is rewarded for completing all the 2013-14 NHL Moments Live. It sounds easy but it can be challenging. First off there are 3 types of challenges:
Be a Pro challenge
You take on the role of a NHL player and have to a certain amount of goals. These aren't too difficult on Rookie, you can easily maneuver between the Dmen easily. The most you have to score that I am aware of is 2 goals with Hertl and Crosby.
Regular Game Challenge
These are somewhat challenging as there are a couple that involve comebacks. As the Leafs you need to comeback from a 4-1 deficit VS Winnipeg and Comeback as the Oilers from a 5-1 deficit to beat Calgary. On Rookie this can still be challenging as the goalie still plays amazing and yours sucks, basically any shot on your net can go in but the other goalie will stone your one timer attempts 80% of the time. I found it easiest to skate around and try to open up the middle so you can deke out the goalie. One timers should still be attempted as well. I found as Edmonton the guys were speedy enough to just skate by the D and get a breakaway. Deke out the goalie for an easy goal. I should also point out that OT Thriller is the easiest moment on Superstar which you will need for the "A Moment of Greatness" Achievement. Game Starts in OT and you just need one goal, if you lose you just restart at OT with 1 goal needed to win the game
Goalie challenge
These ones are pure luck. You play as the goalie and you have to shutout the opposing team with whatever time is left. Like I said it is pure luck. As Goalie you have little maneuverability. You move incredibly slowly and for the most part you are just moving the goalie in the direction of the puck and hope it auto saves. If the AI do a one timer you are screwed because you do not have the movement speed to get over and the AI on your team could not defend against a Peewee team. I did this on Rookie, however some people have claimed Superstar was the easiest (UNCONFIRMED).
thenixhex311 has pointed out that for the Goalie challenges, switching your Forecheck settings to Neutral Zone Trap it can limit the CPU attack time. To accomplish this press left or right on the Dpad during the game to bring up the little menu and press Y until the bottom choice says Neutral Zone Trap 1-4 and press left or right until it says neutral zone trap as well.