i want enemy.TreeHitBox:Destroy() to apply to both of them. But how?

ShellMouthNPC and ForestShellMouthNPC are both alive rn if i run that code it only destroys the object for the first one

if enemy.Name == "ShellMouthNPC" or enemy.Name == "ForestShellMouthNPC" then  --(Wave 8, 10)
            enemy.TreeHitbox:Destroy()    
            print("Attack Removed")
        end

What I meant by my comment was you need to have it one of two ways:

I removed the first one because you shouldn't use it and apparently including it, and telling you not to use it to show you what not to do is wrong?

Now the one you should use is this one, it will loop twice through the if statement you already have:

for i=1,2 do
	if enemy.Name == "ShellMouthNPC" or enemy.Name == "ForestShellMouthNPC" then --(Wave 8, 10)
		enemy.TreeHitbox:Destroy()    
		print("Attack Removed")
	end
end

This last one I didn't mention in my comment because it requires a bit more work, but it will be more helpful to you in the long run than the other two. Now you could get even more fancy and store the enemies in a table and make the for loop even more useful:

for i,v in pairs(enemy) do
	if v.Name == "ShellMouthNPC" or v.Name == "ForestShellMouthNPC" then
		v.TreeHitbox:Destroy()
		print("Attack Removed")
	end
end

i is the number of times the loop iterates (how many times it loops), and v would be each enemy inside the table called "enemies". With this way you could easily check for any property of each enemy by checking for v.Property where Property would be whichever property you want to check for. This also allows you to edit each enemy's property as well.