I don't usually worry too much about what my opponent is going to do and I just play the cards I want to play. It works out decent for me in Brawls, but probably not so well in regular battles!
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Actually, you cannot see the previous lineups in Brawls, and if you do, your opponent likely used different cards than they are about to play with, so Brawls are more random to me :)
I guess I'll end up playing exclusively them soon; once the cards I have will be excluded from Modern games.