Tony Vargas
Legend
Deciding how a monster uses it's powers is an issue every DM has to wrap his head around.Is it okay for a DM to repeatedly attack a PC with an effect that removes him from the combat? Or should he spread that attack around? Obviously I believe the latter. ;-).
Obviously, one way to make tactical decisions for monsters is to try to defeat the PCs - using everything you know about their characters and how they usually play them. It should be pretty obvious that's not an ideal aproach. Only a supremely brilliant/precient/clairsentient monster could know all that.
Another obvious way is to play the monster in the way that's most 'dramatic' or 'fun' for the encounter. It's a legitimate 'storytelling' aproach, but many players get bummed if they realize you're doing it. It's making things 'too easy.'
That leaves trying to play a monster 'realistically,' which is complex. Realistically, monsters spend their whole lives fighting NPCs and other monsters who generally have 1 healing surge and no way to trigger it, and maybe two or three distinct tactical options each. Then, one day, they walk into the middle of a 5-person monstergrinder with a dozen powers each that's able to jump up from 'death' several times in the course of the encounter. Must be traumatic for them.
OK, even ignoring that metagameyness, monsters don't fight determined heroic adventurers every day, most probably do it exactly once in their life, y'know, right at the end of said life. So 'obvious' tactics, like 'gank the mage' are not going to be obvious at all. The monster will know that a given PC can toss a ball of fire when the ball of fire hits him in the nose. He may act accordingly thereafter.
Basically, a monster encounter should be a successions of shocked, surprised, eventually desperate reactions from a monster that's used to roasting villagers armed with little more than pitchforks and snacking on maidens now and then.
A smart monster will obviously regain it's equilibrium faster and start to adapt intelligent tactics based on what he's seen. Run Away, not being the least reasonable such tactic.
Now, setting aside the horror that is PC power levels for a moment, monsters will have some accustomed tactics based on their power set. A monster that can stun an enemy for a turn, for instance, knows that it can either follow up on a stunned enemy while it's vulnerable, or stun another one. It might be accustomed to simply attacking the last enemy that hit it, or the one that hurt it the most, until some other enemy manages a sufficiently damaging attack to get it to shift it's attention. A monster with a (save ends) power, OTOH, is much more enclined to 'spread it around,' since there's no benefit to 'layering' it on someone already affected.
Now, putting it together: A monster is fighting PCs. One of those PCs is vulnerable-looking but lets off a nasy magical attack of some kind, so the monster uses it's stun power on that PC, maybe even follows up to bloody or even drop him if nothing else seems like more of a threat first. But, "sproing" the PC gets back up again. Maybe attacking him is futile? That guy with the holy symbol just tells his allies to get up and they do, so maybe it goes after him next. In the mean time, there's a shouty burly warrior in his face, hitting him whenever he tries to sidle past him or attack one of his buddies. Maybe he should focus on that one. But, wait, there's also some little sneaky guy stabbing the monster in soft vulnerable places even other monsters are too polite to hit you in! Dirty pool!
Bottom line, if the monster focuses on one PC for a couple of rounds, it'll probably take a hell of a beating from some other PC, and shift focus to that one. Add in encounter powers, and whoever he hasn't stunned this round can land something pretty fearsome on him, attracting his attention. Unless the PC he's repeatedly stunning has demonstrated something just overwhelming (like a pre-update stun-lock or a massive radiant damage attack vs an undead), he should be changing targets. Maybe not every round, but often enough to break up the other characters' combos and momentum.