Create a dummy with true-sight (very small effect radius), order the dummy to "patrol" to <target unit> (this is the "follow" command in-game), make sure that the dummy has a very high movement/turning speed & movetype fly with 0 collision radius, then remove the dummy after a second or two.
Or you could try using "Firebolt" instead of "Stormbolt", that can target an area (which you can make the size of 1 unit), and it stuns, allowing you to see invisible units.
Temporary vision of target unit, with very little triggering.
