another spell prob

I got a spell prob... Im trying to make a tracking spell were you getta pick a unit and you see it for a while... well, i think im going about this all the wrong way but Im trying to make it where its a summon spell and it picks a random unit in a radius and follows it... all wrong right, thats where u gett help :foot: oh ya i made another spells its flaming arrows where it light them on fire but im using lighting shield, little did i know lighting shield only hurts the units around it :( so PLEASE HELP.

Thanks :D