Well, a lot of the Defender's fun is in executing those marks. So don't get all metagamey and assume that the monster is going to conserve his HP at all costs, *you* know that the monster is going to lose, and what matters is that he does the most with his limited existance. So, if he has an opportunity to wreak some havok at the cost of a few of his HP's, he should take it. A mark should also not give your PC's carte blanche to walk all over the battlefield. Make them think before they move.
Jay