In a two player game, the monsters keep fighting until one dies. In a more than two player game, the monster can attack any of his opponents, so he might alternate between opponents or team up with another to take out a monster.
We played last night, and a couple alliances were formed. There were two adults playing (me, Wai-ka) and two kids (Axel and Troy). So Troy would attack me, then Wai-Kai, killing my card. I would attack Troy, then Axel would attack Troy, killing Troy's card. The alliances made during the game are not stable though, and there is a tendency to shift sides if one player is too strong.
It reminds me of the reality show Survivor, where people are teams until the end; then it becomes every man for himself.
We initially played with a piece of paper to track scores, but found that since it was subtraction only that we could keep the scores in our head and play faster. If you forget your score, your opponent will happily remind you.