First of all, you have to make an initiation or invokation of the spell: How is it cast?
1, Is this a targetable ability like Firebolt, Slow, Holy Light?
2, Is this a buffing ability that will apply the effect on basic attacks?
3, Is this an ability that is cast to a location and searches for units in that region?
etc.
Then you have to take on the logic of what you have to do:
- If (Target has less than or equal to "175 + (75 X level)" health) Then
---- Kill the target.
---- Reset ability cooldown.
---- Give bonus movement speed to nearby allies.
- Else
---- Deal damage to target
None of these things are any problem at all.
You can use something like bonusmod to apply movement speed bonus (or use a normal ability).
You can remove and add the ability to reset the cooldown.
Killing the target via the GUI action "Unit - Kill <unit>" is a bad idea. Instead give it a 0.1 seconds generic expiration timer.