NBA 2K10

Ok since they patched this, it really isn't that difficult. So start the game and choose your player. I chose Lebron. And for the next 3 players make sure they are retro players, or just players that aren't superstars. The reason being is because there are challengers like T-mac, Kobe, J Richardson, and I don't know the rest. So to be safe pick 80's or 70's players.
Then control every player. Pick your fist player to win it all. When you start off, do a simple dunk. When it comes to the next three players, do a simple dunk but don't press RT. you will miss the dunk. Do it again with your replacement dunk. This will save you about 15 seconds of time for each of the three players. And trust me it adds up. Keep going until your first player wins and your other retro players get knocked out.
Now if you're lucky the challenger will come. I've heard it said that it's random, but for me he came after every 4th time I won the contest. Now this is where the tough part comes in. In order to get enough points to win, which is in the mid 90's, you have to place a prop, throw an alley hoop, jump over the prop, and then throw the hammer down.
The worst part is that only certain dunks work over certain props. There are only 2 dunks in the dunk-off, so here's what I did. My first dunk I placed 5 tires, and then threw the ball and pressed down as my gather, and then down again as my finish. I got a 94. T-mac was my challenger and he got like an 89 or something. Then for my last dunk I placed a construction rail and pressed left as my gather and then right as my finish. I got a 91 I think. I ended up winning by 2 points. Which was probably lucky. So you should probably practice different dunks with different props, and then try them out in the dunk contest, before you attempt the challenger. Ok this is it. Use this strategy to get this pre-glitched achievement. I hope I helped adding to a future 1k.

Defeat any challenger in a Dunk-Off.
This is one of the harder achievements in the game. From the main menu, go to Game Modes and then NBA Blacktop. Once there, select the Slam Dunk Contest. Challengers sometimes appear at the end of the dunk contest, after you win. The announcer will finish saying that you won and then someone will pull up in a car and come out and challenge you. They appear at random times, though.

There are five people that can challenge you. Kobe Bryant, LeBron James, Tracy McGrady, Vince Carter, and Jason Richardson. It is best if Tracy McGrady challenges you because sometimes there will be a glitch where he just stands there and doesn't dunk. The glitch only happens with him but no one knows how to set it up, it just happens sometimes.

Before you start, you will want to decrease Tracy McGrady's stats. Lower his dunking, vertical, and height statistics to 0. This doesn't always help, but sometimes it does so you might as well try. Thanks to Actiionz for this tip.

When you select your players, pick Bryant, James, Carter, and Richardson. That way, if there is a challenger, it will have to be McGrady. Once you start, only use the dunk with up on the for both the gather and the finish. (The gather is the first move and the finish is the second.) Make every dunk and it doesn't matter who wins. Once you finish the last dunk in the final round, don't press anything. The car should show up but it might not. If not, just do it again.

Once the challenger does show up, you will have two dunks to get a higher score than he does. What I did was put 3 speakers down in the center and then use right and right on the for the dunk. You should get a score in the mid-90s. Then for the second dunk, I put a car in the center and used down and down for the dunk. I beat him the first time I used those two dunks but I had 3 challengers before that and I messed up on alley oops on all of them. If you use an alley oop, it will increase your score but there is a much greater chance that you will mess up so only use it if you know the timing perfectly.

This is off Xbox360achievements.org
thx Pashos

I have a feeling this achievement is glitched. I'm not 100% sure, but I have beaten about 3 competitors in a dunk-off and still no achievement. To beat them, this is what I did-

1st dunk- At about the 3-point line, rotate the right thumbstick counter-clockwise twice. You should do a cartwheel-windmill dunk. It should net you at least 93 points.

2nd dunk- Set up the short bench right in the middle. Then, a bit before the 3-point line, rotate the right thumbstick clockwise twice. You should bounce the ball and pick it up and jump over the short bench with no problem. It is almost always 100 points if you get it on your first attempt.

Since I have no achievement to proove that these tips work, I can understand why you wouldn't trust my guide. I hope it works for you guys!

*Edit: You have to average about 90 during the regular competition to get a dunk-off. To get there, just do what the other guide says and throw yourself a pass and jump over any prop. They are almost always 90's.

Alright so i just completed the game and unlocked the achievement as of 1/15/12. Now what i did was i controlled all the players(4 players). I chose players from the 70-80 era. I played through all the rounds doing the same dunks, making sure my first player was able to progress to the next round(doesn't need to have to be 1st every round). I was scoring bout 60-80s on my dunks. Then when the 3rd round is over, you can press A to skip the talking and crud. Then a car should drive up and a player will challenge you. Make sure to have more points than the challenger at the end(there are 2 turns each). The challenger appears random. Against the challenger i recommend top full counterclockwise x2. Then on the next turn do top full clockwise x2. Make sure to make the dunk on first try as i almost lost but won by 1 point!!!