I take long time cuz VM aren't notifications i get on my phone, so i don't receive them instantaneously,
you don't get it hein...:
An item (powerup type)
Give it an item ability that gives 30 hp ok?
and create a similar one but this time it gives -30 hp (SHIFT + Double Click to add negative values)
When you give the first item to the hero, he gains 30 hp, and when he gains the second: -30 hp
So, the 4th spell has stacks that give 90/120/150 hp or in other words : (3x30/4/30/5x30) so to add a stack on level one, you will have to give the item 3 times to the hero (create 3 items and give them to the hero) , to remove a stack on level 1, you will give him 3 items of the -30 hp
Get it?