I've read the rules. I know the math. I know, as the player, that X monster is vulnerable to Y damage and if I move through the door 4 squares into the room, my burst 2 will catch every baddie there. But in the 6 seconds since combat started (he's still surprised!), he failed his knowledge check and he doesn't know whats around the corner.
Suggestions for how to play with that knowledge gap? I've almost caused a TPK by not playing the most tactically beneficial way because I didn't believe my character would have figured it out yet.