![Image](http://i.imgur.com/szXrz2o.jpg)
(Disclaimer: This strategy is not 100% guaranteed to succeed due to the RNG of Medivh cheating. However, it is the strategy I've had the most success with in the past when soloing the Chess Event. I don't know if the nerf last night makes another strategy more viable)
I can not state enough how important it is to get your King out of the fire. Stop whatever you're doing and get him out when Medivh cheats.
Not mentioned in the pic but still important: if you do choose to use your king towards the end, keep him close to the rooks. And while it's stated you can choose King or Queen at the end, the reality is as you get more comfortable you'll want to move some more pawns out and possibly reposition the rooks or knights to deal with the opposing pieces as laid out by Medivh.
Good luck!
EDIT:
NOTE: I'VE REJECTED THE ALTERNATE STRAT. SEE PAGE 3 FOR INFO.
An alternate strat I'm going to try later since it appears Medivh doesn't cheat as fast as he used to:
1. Move out King's Pawn (as normal)
2. Move the queen out two spaces in zig-zag (so she ends up in front of her pawn)
3. Continue with strat as normal
Getting the queen out may mean earlier damage on the opposing king, and will allow you time to move the queen also if the king and queen are both targeted by fire (in the original strat if the king and queen are both targeted, the queen is essentially sacrificed)