How can I win against Phyrexian Unlife?
The full card text is:

You don't lose the game for having 0 or less life.
As long as you have 0 or less life, all damage is dealt to you as though its source had infect. (Damage is dealt to you in the form of poison counters.)

If you have ten poison counters, you lose the game. You just need to get your opponent to zero, then deal ten more damage to him. He'll take it in the form of poison counters, and lose. It will buy him time - maybe he was almost dead and suddenly you need to get him to 0 then deal ten more damage - but he's not invincible.