The key is to not to rely on it. Merely prioritize application and watch for it to drop, maintain the pressure. This ability does not offer suddenn results. Also, anything you can do to reduce their saves, curse as a primary example, is crucial to combine with your minister.
When you see it is landed, proceed with your normal combat strategy. Do not expect it to lag them, use every other tool at your disposal to prolong the fight. The more tics you are in combat with your foe, while they are afflicted, the greater chance you have for a game changing 4 round lag.
Also, remember, minister cannot be dispeled. This means you can freely dispel your enemy. Applying minister, then pathing...just to try your dispels (success rate will be equal to your mnister success rate in mist cases) right before the tic can really ruin a giants day.
So dont rely on it, never stay in combat below your flee/heal threshold hoping for that lag. Just keep yourself, and try to direct the battle, into a position where if that lag does hit you can quickly annihilate your foe with raw unsancec paths.