The image above is from the earthquake game mode. This game mode causes 2 damage to all none flying monsters i.e. monster without the flying symbol below them:
As you would think using flying monsters in these game modes gives you an advantage. What is almost pointless in using in these modes are monsters with low health and none flying, since they die after only 1 turn.
In a recent announcement it was stated every week or two a new game mode will be introduced and these new modes would come up more often than others while they are new. This reminds me a little of hearthstones tavern brawl, a new mode every week.