Just simple create a trigger with an even of EVENT- EVERY 0.05 SECONDS
Then add the unit to a group...or you can directly create a UNIT-VARIABLE...set Unit=Your unit..
Add the IF THEN ELSE.....IF REAL(MANA OF Your Unit is greater that or equal to (Mana Required)
Unit- Create a dummy to the position of Your Unit
Unit - Add Cripple to the dummy
Set the cripple slow percentage to AS-0% MS-0% DAMAGE REDUCTION-0 Targets Allowed AllieS,HERO,GROUND,AIR,FRIEND
Cripple for me can create a buff without hurting your unit..
So you can also change the buff of Cripple to your want...
But the trigger is really depend on where you will use it...