i don't really agree with allowing the attacked player to use temp hp.
i think that finding targets for attacks is enough difficult as it is, without having to worry about finding some with +2000 hp.
also, the attacking player has a lot more to lose than the attacked one.
on the other hand, i don't think it would be wrong to limit the temp hp used to attack.
baluski's idea could work, but what happens when that guy dies? but it also makes a players strength count less, against a large number of enemies