Space Jam: A New Legacy - The Game

Other guides are great, but here is a cheap way to score a stupid amount of points (good thing there isn't a global leaderboard, or this would break it).

Play on any mode/difficulty.

Get all of the way to the last boss.

Play through normally until he drops the green tank that you must destroy.

Grab the ball, but don't charge it.

Stand on the far end of the screen and throw the ball at the tank.

When it bounces to you, punch it. It will give you 100 points and then bounce back. Punch it again when it comes to you. Each time you punch the ball, it will award an extra 100 points. (so after 100 punches, you would be getting 10,000+ points a hit).

Just keep this up until you get the 999,999 (or until you get bored). Not sure if there is an in-game timer that will eventually stop you though.

Note: If you are having trouble understanding this via text, here is a clip:

Please note, this is not the original clip. It was removed for some reason. This one is a bit crude, but should get the job done.

very easy to farm first beat the game to unlock Boss rush Mode then beat the first 2 Bosses then let you kill on the last Boss you can try it again but you will save all the points you get before do this until you reach 999999 points and you have this Achievement

You can easily complete this in Boss Rush mode, which is unlocked after completing the game. You will face the three bosses from the game, without any enemies in between. Defeating each awards 100,000 points. So in order to reach the required score, you need to defeat the first two bosses and then die to the final boss and select Retry. You will spawn at the beginning of the Boss Rush, but keep all your previous points. So just rinse and repeat until the achievement pops.

2 controller method:

Start a new game with 2 controllers, finish the first wave of robots, then position yourself either end of the screens facing each other whilst holding the ball Press to throw the ball to the other person and before it gets to them use the other controller to again press .
This should result in it being punched back to the original thrower, keep punching it back and forth to rack up the required amount. Took me just under 5 minutes and I dropped the ball a few times.
If you do drop the ball, Tweety will reposition it in the centre of the screen. Rinse and repeat the method above.
This is going on from another guide here but don't have to make your way to the end boss so saves lots of time

There are a couple ways of doing this. 1. Completing the game normally, and then on the final boss fight locate the blue capsule with the save file inside. Simply hold to summon the ball and then press to throw the ball. When the ball comes back at you, press to hit it back at the capsule. Repeat this process until you receive 999999 points. 2. Play the boss rush mode, and defeat the first two bosses. Die on the final boss and you should have your points still with you. Keep doing this method until you reach 999999 points. If you have done any of these two methods the achievement should have unlocked.