NHL 11

100 games isn't hard, it just takes a while to grind out. On the other hand, getting 100 games while getting good grades isn't easy. The amount of games is cumulative between your Skater and Goalie, so playing 90 as a skater and 10 as a goalie will net you the achievement. Grades unlock higher cards, which in turn unlocks more XP which allows you to increase your attributes.

The grading system
Position Score: This grades on you on how well you can stay where you are supposed to be staying. Sometimes during the game you will notice a blue arrow pointing to a spot on the ice, this is where you should be to help out. As long as you are following it, an A- is easy to get. You will be penalised for being out of position, but being caught out upfield (in your opponents defensive zone) is much worse than being caught out downfield.
For Goalies, you should always be in the net, except when you are being pulled for a delayed penalty or going out to collect a slightly threatening puck. If as a Goalie you are not getting an A you are clearly doing something wrong.
Team Play: As a skater the following will add to your team play score:
Connected passes made, Successful loose puck dekes, Takeaways, Blocked shots, Penalites bought, Icing avoids, Clean hits, Winning faceoffs, Winning fights
The following will subtract to the team play score: Unseccessful loose puck dekes, Giveaways, Penalities taken, Missed shots, Offsides, Icing, Losing faceoffs
For Goalies, Team Play is a little weirder. A goalie should never take a penalty (handling the puck in a resticted zone). Secondly, if the option to cover, you should only pass if it is clear, or you are absolutely desperate (e.g. 1 goal down with seconds to go), other than that keep freezing the puck. You should always freeze if you hear someone shouting something like that. Other than that Shutout Periods and Desperation, Penalty Saves seem to keep the score up.
Stats: Affected by the following: Goals, Assists, Faceoffs, +/-, Good shots, Hits
For Goalies: Saves, Save Percentage, Goals Against Average, Shutouts

The next bit is quite complicated, forgive me. Your Skater and Goalie cards are kept seperate, any games and grades they get do not mix between each other. The overall grade is a form rating from your last few games (I believe it is 10) and is an average. If you want to unlock the higher cards for more XP you need to keep the performance up. Quitting out of a game counts as an overall F and will gratly hinder your chances of unlocking the ledgend card.

Rookie: C- 10 Games
Pro: C 30 Games
Veteran: B- 60 Games
Superstar: B 100 Games
Ledgend: A- 150 Games

There is an oddity however, if you unlock the (let's say) Veteran card, you will get the XP for both your Goalie and your Skater, regardless of who you unlocked it with and if you do manage to unlock it again with the other character you will not gain any more rewards.

The other way to unlock attribute points is of course boosts. These can be purchased for Microsoft Points or unlocked by playing EASHL and Be A Pro.

A game usually lasts about 20 minutes, but the time becomes extended due to penalites as it counts down slower, so try not to take any penalities and make sure you clear up your Powerplays quickly. Not only will the game finish faster, but you'll avoid any hits to your grade.

***EDIT***
Servers for this game shut down as of January 11th, 2013 so this and all the other online achievements have become unobtainable.

Boostable with a minimum of 4 people and a maximum of 12. Create two EASHL teams, then divide the teams up evenly and have half join one team and half join the other. Now have both teams go into the EASHL dressing room (Play game from the EASHL menu) and have everyone select a position. Have one of the team captains hit the right bumber and type in the name of the opposing team and challenge them. The other team captain should receive the challenge in the bottom left corner and should hit left bumper to accept. Everyone ready up and select a position let the game run for two in game minutes and then have someone quit out. Rinse and repeat 99 times. You then have to play a full game for the achievement to unlock. You can always check how many games you have in the performance tracker under My Pro in the main menu. The other solutions are also correct but I believe this is the quickest way to get this done. We were going at pace of about 25 games an hour. (4 hour total).

Play 100 Ranked Matches with your Online Be A Pro.
This can be done by playing Goalie, or Skater, and can be done in the following game types.

-EA Hockey League
-EA Hockey League Drop In Games
-Xbox Live Community Games

Easily boosted by playing past the 18 minute mark then quitting. It will count as a game but you will get all F's for stats, which will make getting your player cards next to impossible.

Play 100 games online with your pro in any combination of Drop-in games, EASHL club games, or Online Team Play.

Basically, unlock your Super Star card and you'll get the achievement.

Note that leaving a game generally does not count towards a game played.