Its not something that would take less than a minute by minute diary of my life to prove but I will swear that I occasionally have less than lucid dreams where what happens will match up with what I experience later. It seems a lot like your idea of a precog but it is never anything significant and I can't tell if any given dream will match life or not. I often have no conceivable foreknowledge of the conditions either. One was of myself walking down a path in indonesia more than a couple months before I was in the country. The poison idea is ingenious but could be noticed if the precogs had random other visions or visions in general. A lot of it will depend on how they percieve future conditions. Do they just know? Do they experience it? Real time or faster time? Is it a spidey sense of unease?
As for what I would suggest. Things can only consistently work if the precognition is limited. Particularly to their point of view or one close. How do you keep them from knowing that their ability is going to be used against them? I would engineer a situation where the leader actually gets saved but the saving process puts him in an longer road to a definite capture. They may be able to foresee their own deaths, but if the leader is saved this won't necessarily matter to the same degree. If its hard to recognize a precog situation or tell how they really will play out or they take time to experience, you could attempt to "spam" their mental inboxes, especially with someone on the inside to monitor things. The more work they have to do the harder it will be to do it. Superhumans are still human.
