Kept trying to complete the same stage over and over. Its beneficial (and necessary to some extent) to capture the monsters instead of just killing them. The problem is that the boss monsters will only stick around for a certain number of rounds before they take off. This means you have to fight through all their minions, get to them, cancel their invulnerability, reduce their massive hit points, and then try to get their hitpoints low enough so that their is a mild chance to do enough damage in one hit to beat their massive overkill scores, all very quickly. And then when failure finally comes, typically because time ran out or the overkill blow wasn't powerful enough, the player gets to spend another 30 to 60 minutes doing it all over again. Or the player can give up and move on, as I finally did after six or seven tries.