Xarlen said:
They have delt with a oslyth. It was invisible, called up some walls of ice, illusions, dretches, and they hacked through the walls, before splattering it.
The bat can simply sit in it's master's pocket, and poke it's head out, 'looking around'. Considering it's 10 intelligence, it can relay the information pretty well. 'It's humanoid, standing <insert location>'.
I want to respond to these two points -- I think they might be illustrative of some things you can change to make your campaign more challenging for the players.
With the osyluth, you note that it can teleport without error at will, right? Unless there's a very good reason for it to fight to the death, it oughtta blip on outta there when the going gets too rough. It's not terrifying in a mano-a-mano fight, but it's got great subversive powers it can use behind the scenes.
In general, when you work with a monster with funky powers, spend some time brainstorming cruel and unusual uses of those powers. Consider how it would use the powers outside of combat. Consider how it would try to cover up its weaknesses. Consider how it would try to escape a losing battle. You may find that enemies thought about like this become much tougher than they've been previously.
Second, regarding the bat -- I invite you to try an experiment, involving two friends. One of them gets a long stick with a 6-foot piece of rope tied to it. The rope will represent an invisble opponent. The other friend stands nearby. This friend represents a bat familiar. You get a sword-sized twig (make it a twig for safety reasons -- a wiffle-bat will be better).
Close your eyes. Friend #1 dangles the rope somewhere near you and moves it around a lot. Friend #2 shouts directions to you to help you find the rope. "To your left! I mean right! Forward -- not that far forward! It's moved behind you now! Look out!" etc. You try to hit the moving rope with your twig, keeping your eyes closed.
You're being awful generous by allowing the bat to communicate to the wizard where an invisible opponent is. At best, I'd let the bat communicate a set of four squares; the wizard needs to choose one of the squares to attack, and if he chooses the correct square, he still suffers a 50% miss chance. Other PCs can witness the space that the wizard is attacking and come to their own conclusions about where to aim for.
The bat should, of course, be able to tell the wizard that something isn't showing up to blindsight. Although I'd allow a spellcaster that's experienced blindsight to create an illusion that would fool blindsight: a combination of tactile and sonic illusions oughtta do this effectively. If you do this, let your wizard know that he could probably create an illusions that would fool your bat, now that he knows how the bat "sees" things; that should be the only clue the wizard needs that this power isn't foolproof.
It sounds to me as if your players are interpreting the rules very favorably for themselves and that you're acceding to their interpretations. Given their maturity, I think they're likely to agree with you that their interpretations are skewing the game, and that there are other, more reasonable interpretations that y'all should start using.
Daniel