You vs a single target...
You would want something of a buff.
So you kill them faster.

You vs a group.
You would want some kind of a CC effect.
So you can isolate and pick them off.

Unfortunately, you cannot control what you get.
If the casting time is very fast or instant.
You might be able to land it, when you see their cast bar doing a nice spell.
Not sure on that at all though.