Odo gets knocked out a couple of time, firstly (As @KyleJones has pointed out) in DS9: Vortex by what appears to be a blow to the shoulder. At this point he's in changeling form.
But as they pass through the tunnel, there's a direct hit and rocks come sliding down on top of Odo...
CRODEN : Odo!
He leans down and examines Odo, who is unconscious...
It's not especially clear what caused his injury but from reading the script, it seems likely that the writer originally intended there to be a lot more rocks!
He also gets punched out (by O'Brien) in DS9 : The Assignment but this is whilst in human (non-changeling) form
As regards phasers, there doesn't seem to be a setting that causes unconsciousness in changelings. In DS9 : Homefront we see them testing a "phaser sweep" that they think will force a concealed shapeshifter back into their liquid form and may prevent them from escaping.
BENTEEN : Three point one. If we set the phasers at three point four, we should be able to stun any Changeling we hit and force them back into a gelatinous state.
ODO : I would make it three point five just to be on the safe side. But if you want to do more testing, you'll have to find another guinea pig. I've been shot quite enough for one day.
At higher phaser settings, changelings simply explodeexplode
