As the DM it’s my job to control all of the monsters during a fight. Each player runs his own character but everyone else involved in the battle is my responsibility. In some cases the Monster Manual provides tactics (albeit very basic tactics), but in the vast majority of situations it’s completely up to me to decide which monster attacks which PC and what power they use.
As the DM I have to decide if the monsters are going to do what’s most tacitly sound (basically, what’s best for the monsters), or are they going to do what seems most fair to the players at my gaming table? For a long time I’ve been doing what’s fair and paid little attention to tactics. But the more I’ve been thinking about this approach the more I think that it’s hurting my game.
D&D isn’t (or shouldn’t be) the DM vs. the players. It’s a cooperative, story-telling experience with a lot of thrilling combat thrown in. Although we often joke about winning D&D when the PCs defeat all the monsters during an encounter, this is obviously not the case. Yet if a PC dies during combat the player certainly feels like he’s lost the game. For this reason I generally try not to pick on one PC and have the monsters gang up on him. After all, no one like it when their PC dies. But am I really doing the players any favours by not having the monsters employ sound tactics?