First of all, most users make the mistake to set-up a rainbow team against Kashhrek (in tournaments and in raids, too). Therefore your first move is to change this set-up.
Use two, three or even more reds. Take a dispeller with you if you have got one in your roster. He/she will make it easier for you to win against that lizard guy. Usual dispellers are Melendor, Sonya, Caedmon and Sabina. All four of them are available in this tournament.
The strategy to avoid attacking Kashhrek like @Scarecrow already describes above is a great one, especially in a very fast tournament (which is it at the moment).
There are a lot more other tactics you can and should use. Mix your team and try some synergy between your heroes. There is no hero in that game who is unbeatable or nearly unbeatable (even @Olmor s two Mother Norths will lose from time to time). You just have to use several heroes together and several tactics. The balance of this game is nearly perfect so that you can beat every enemy with a lot of combination of your heroes. Just try it and don’t givew up to soon.
Enjoy the game, beat Kashhrek!