Odo has demonstrated that he can at least pretend to be drinking out of a rigid glass mug when the mug is in fact part of his body, so I'll give him the ability to manifest a knife or something.Metrion Cascade wrote:- a shapeshifter who can form metal objects and sharp edges (the burden of proof is on anyone who says he can't do both at once) and form restraints, barriers, and animals (all shown onscreen), and has no blood for the bats to smell, and can probably form anatomy capable of seeing infrared or echolocating (there are Earth animals that do each), and can't be injured at all by the bats.
His ability to imitate something capable of IR vision or echolocation would presumably depend on him being familiar with such a species. Maybe there's something appropriate on Bajor...
He can be injured. If a large bat-monster were to, say, bite off one of his arms while he were in his humanoid form, said arm would liquify inside the monster's body (as demonstrated by the "Founder test" devised by the Klingons, spill some blood from a Founder and watch it turn to Founder goo). I don't believe Odo has demonstrated the ability to control portions of his own mass at a distance, so I'd say that "arm" is lost to him. It may be indigestible to the monster, but it's still gone. If swarmed with the bat monsters (a very real possibility in this scenario), Odo could very easily be nibbled to death.