So you want to change the damage dirt does to a spider, or something like that?

Code (Skript):

1. on damage:
2.     attacker is a player
3.     player is holding dirt
4.     name of player's tool is "name" #do you want it to be an specific dirt with a custom name? if not, remove this line.
5.     victim is a spider
6.     damage the attacked spider by 3 hearts #If I remember correctly, it won't change the original attacker and will count as if the player killed the spider if it dies by this damage. I'm still not sure.

Edit: Edited code a bit

 

I want to create a laser, so when i click with something its fire a laser beam and damage the target entity
Full code:

Code (Skript):

1. on leftclick with steak:
2.     if player is op:
3.         if distance between the target and the player is more than 10:
4.             message "too far"
5.         else:
6.             loop all entity in radius 1 around target:
7.                 if loop-entity is monster: #this line is not working
8.                     damage the loop-entity by 3
9.                     drawLine particle redstone, center player, target location of target, id "%player%", rainbowMode true, solid true, density 20, length 10, zigZag count 0, height 0, visibleRange 32, displacementXYZ 0, 1.3, 0, pulseDelay 1
10.                     wait 2 tick
11.                     play sound "ENTITY_VEX_HURT" to player with volume 5 and pitch 3
12.                     stopEffect id "%player%"