Side note: You can also 'adjust difficulty' by creating massive auras that affect whatever units. Just set the range on the auras to be maxed out. And make the unit not exist (locust/no model/invulnerable). With this and negative or positive integers you can easily change attack speed, damage, move speed, life/mana regen, and armor.
PLUS: Another trick is that it give all the units a buff- which you can trigger all units WITH that buff to have less or more hp, etc.
EDIT: And for level adjusting just make the trigger change the level of the ability for the unit.
(Details on request)