It's more or less random if you win. The closest I came to consistent wins was to control the two healers (warlocks?) and keep the king healed to full. If you can get the other king down to half with yours still at full or even and your healer(s) still alive, you'll win. Tough to do, though.

[edit] One thing you MUST do though, is clear some space for your healers and king to move around when Medivh "cheats." If you can't move out of his fire, it's just about a guaranteed loss.