How can you injure an opponent ?

You pretty much just beat them as bad as you can. It's fairly random, but there are some ways to help it along

1: Make them Rivals. Most times it's tied to that
2: Extreme matches. (like Extreme rules, HitC, TLC, ect)

Lastly, this can only be done in universe.

If you are in a normal match with normal match rules meaning DQ is on and count out is on, try hitting the same body part as much as possible, start off by using simple punches and kicks before grappling because they counter grapples pretty easily as a start, after that try to start using grapples and use some grapple moves into the corner.

If it's no DQ then use weapons, lowers their health pretty easily.