Odo gets knocked out a couple of time, firstly (As @KyleJones has pointed out) in [DS9: Vortex][1] 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...**

![enter image description here][2]

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][3] 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 [explode][4]


  [1]: http://www.st-minutiae.com/academy/literature329/412.txt
  [2]: https://i.sstatic.net/f5dGh.gif
  [3]: http://www.st-minutiae.com/academy/literature329/483.txt
  [4]: https://scifi.stackexchange.com/questions/20410/what-happens-to-odo-when-he-is-hit-by-a-phaser