...or What We Learn From Zelda: A Link to the Past.
Well-designed solo monsters for D&D 4E flip out and gain nastier attacks when they are bloodied. Even some regular enemies do that - my favorite example would be the angels, who as a rule drop their angelic stoicism and go bananas when you've hurt them enough.
Zelda: A Link To The Past had an interesting twist on this over a decade earlier. There are certain bosses that are really separate creatures, and when only one of them remains, that one gets a serious boost in speed or damage output. The last Armos Knight gets faster, the last Lanmola shoots more rocks when it emerges from the sand*, etc.
*) Not that it's not still one of the easiest bosses.
What this encourages is - at least when fighting the Armos Knights - to spread out the damage a little between the creatures. That last Knight is a royal pain in the behind, so if it can be damaged before it goes nuts, so much better. That's obviously more pronounced in a 4E fight where an individual monster takes four hits to kill rather than two (as in Zelda).
Also, 4E has encounter powers and dailies. Having the last man standing get vastly more powerful encourages saving the more powerful attacks to kill that one quickly. Obviously, this needs to be telegraphed in advance, maybe by boosting the remaining monsters slightly everything something dies (this requires at least three monsters, of course).