Define xDamage globally, as an array, sized using MAX_PLAYERS + 1.
In xOnDamage, you would do xDamage[ id ] += read_data( 2 ).
Set the slot value to 0 on client_disconnect(). Or at new round if that is when you want it reset. If you want to reset everyone, you can use arrayset().
sorry i'm very new to this script business and i didn't understand very well