Injustice 2: 7 Crucial Pro Tips The Game Doesn't Tell You

A cross up is a technique that entails tricking the opponent into blocking in the wrong direction.

In short, a cross up is generally performed by jumping over an opponent that is blocking, and hitting behind them. As the opponent is blocking in your direction before you jump, there is very little time for them to correct the direction they're blocking in before your attack can connect.

In Injustice 2, it's possible to combo into an aerial attack, so a successful cross up can lead to massive damage.

Of course the difficult bit comes in making sure your attack lands behind the opponent, instead of directly in front of them. However, this is a little advanced for newer players to get down, so it's more important to first learn how to defend against a cross up attack.

It's usually much more difficult to properly block, so instead simply try to back dash away from an opponent who looks like they're attempting it. You could also try punching them out of the sky, but a character like Brainiac can perform a special divekick which can easily break through your attack.

In short just pick a ranged character and spam, you'll have more fun that way (probably).