In event 5, create a text object with the fade behavior at enemy, and set text to damage amount.
event line 5, 1st action, and 8, 1st action, one for the player turn and one for the AI turn, spawn object text at the respective enemy/player character XY or imagepoint and set the text value to the enemy.dmg or player.dmg and as oosyrag said, having a fade out behavior on the text object will make it disapear after an amount of time, you can also set the bullet behavior and set angle of motion to 270 when text object is spawned, and make sure to untick the "set angle" from the text.bullet behavior default settings everything else you can leave as is.
if you do it corectly, you should have a text object popup on the damaged party that will say the dmg done and the text will go upwards while slowly disapearing over 1 second as the default value of fade behavior is.