I think that unless PC's had experience dealing with any monsters knowing what their lowest or highest stat is would be more metagaming than anything, unless they rolled a requisite skill check. If my PC's use this tactic I just switch defences around, making that minotaur particularly dextrous to help his reflex.
Well the thing is that a skill check doesn't tell you much more than trivia about the monster and maybe a certain attack to avoid or that you should use fire or something, they won't tell you which defense to target. It's not metagaming to target a zombie's reflex save, and avoid an ogre's fortitude save, these things are obvious. The main problem is that many monsters have no obvious low defense, or a non-intuitive one. That's when the wizard has to turn to magic to learn the answer (using up a power). It's not fair that against half the creatures in the manual, the wizard has to take a wild guess at which defense to target, getting -5 to hit if he guesses wrong.
It might be a bit powerful to know the enemies weakness, especially against a major villian?
Maybe. Knowing that one of the villian's defenses is lower than the others is only effective for about +2 or +3 to hit for the wizard/warlock and maybe one other party member. Chances are if the villain is wearing full plate the wizard knows to target reflex, and if he has robes and a wand, the player will target fortitude. So the spell isn't giving much more information than might be available. The villian can also adapt from encounter to encounter, using items or spells for protection. It's not game breaking, it just gives the players a bit of an edge.
My PC casters try things out and then remember them "Oh this guy seemed to have a low Fort, I'll try that again" esp. if you run into similar monsters in later encounters (see also: KotS).
But to get an idea of what a monsters defense score is, you want to hit it with a few attacks (3 or 4), and to do that for each defense is painstaking. Yes, maybe an '11' hit the kobold minion's will save, but that doesn't tell you much about the dragon shield or the shaman. I'm not saying that your players shouldn't remember which attacks work best on who, I'm just sayin that they don't have enough attacks or luck against some monsters to do it all the time.
Isn't that what the Monster Knowledge Checks are for?
Monster knowledge checks often give no insight into which defenses are lower, and sometimes just guessing will put you at 5 or more points less to hit than another defense. This desparity is often not obvious from the monster's description. Would you guess that the otyugh's will is 3 points higher than its reflex? I would have guessed high reflex and low will for an animal like creature that uses traps and stealth.
That and, when you miss an attack versus XX with a 19... you try a different power.
Absolutely, sometimes you get lucky through trial and error and learn a little bit about a monster's defense. However, having to attack the creature with several innefective spells before you land a hit is annoying, especially if the characters around you are hitting with weapons more often. I made Libra because many monsters have low defenses that are very very difficult for the players to ever learn of. By the time you figure out that it is best to target the
Spectral Panther's will defense, the DM has already thrown 4 of them at you, and has moved on to other monsters.
Another cool thing about libra is that the players can collect information on certain monsters in a beastiery sort of format (in the style of final fantasy et al.). Of course, you can always throw a monster with varied defenses at them to keep them on their toes.
