ok, i think the best way to explain it would be like this. Imagine each enemy would have two health numbers. ONe is 'real' health, the other is 'chrono' or 'temporary' health. so you have a creep, like so.
Creep: Health: 500 Chrono health: 500
The chrono health is affected by real health, so if it takes 50 damage from a turret,
creep: health: 450 chrono health:450
Now lets say it runs into a chrono turret, which does say 15 damage per hit, second, whatever. The enemy is frozen and the real health is invulnerable, but the chrono health is affected, so now
Creep: Health: 450 Chrono health: 435
As the chrono health is slowly whittled to zero, the creep takes no real damage. Once the chrono health hits 0, the creep is removed. However, say the LOS to this creep is broken by a flak shot from a nearby turret hits a different creep nearby. the chrono health is restored to what it originally was.
Creep: Health:450 Chrono health: 450
So its basically an all or nothing turret. Bullets and the like wouldnt affect the LOS, although explosions near it may break the concentration. That could be one rather simply by making a sort of unit that is basically a line from the turret to its target, and certain types of attacks will break it.