I've proposed something like that for a poison rule in another thread. The problem for an evocation attack like this would be. However, you would not (at least I would not allow it) get a bonus from an attack roll, so it might or might not even be enough to take anything out.
Let's say you summon enough power to get off an attack with 8 shifts of power, the ghoul takes of 4 for his athletics, leaving 12 shifts of damage.
The same attack done over time would look like this: to get the most out of it, you would split your shifts evenly, so weapon:4 for 4 exchanges. The attack roll would only be made to see if you can land the spell, it is on his own from there on. so that would be 4 attacks at weapon 4 against endurance (+3). That would boil down to roughly 1 stress each exchange and therefore 4 stress total.
Of course you could do it another way, but this is how I would do it.
The best way to do something like this would be to stack maneuvers onto your target to tag them all for one gigantic attack to knock him out. Those maneuvers could still be described as an attack, in this case the particle accelerator, but mechanically they would not do any damage until the actual attack.
The other spell was an idea to allow the wizard to down one mook a round for several rounds for the cost of one mental stress. Basically, the wizard would summon up the destructive energy and direct it through several exchanges.
This would simply be an offensive block, and blocks can be increased in duration without a problem. You can even put more energy into it, if you do it in an exchange, where it is still up and running.