so essentially demons can control a liquid that can have a density of up to 1500 kg per meter cubed (which is just about less dense than rubber) when being controlled. luckily demons have the magical ability to control it with the fluidity of a water bender from avatar. so sending massive meter wide flying fist of this liquid at full density, 40 meters per second at a demon's chest, would most likely break or kill them. maybe. but demons have had this magic for ages, and thus have ways of using their magic to protect them. they can make a full density void armor, and under the armor is a 3 inch layer of just foam-like, squishy void. (some demons have the ability to set properties to void in order to not have to constantly focus on every air pocket, or every atom) doing this they are able to resist things like that with this protection. how strong would it be if it was resisting those punches?
This stuff would not be worth much.
I take the question to be "how well would a magically controlled fluid of mass 2kg/cubic meter protect a demon from those punches". "Those punches" I take to refer to a "meter wide flying fist 5 meters per second".
5 meters/second is 11 miles per hour so more of a firm shove than a punch; Google states kung fu punches are about 40 miles per hour. For either one, the fluid is the mass of a heavy gas, air being 1.3 kg/cubic meter and CO2 1.8 kg/cubic meter and so I think this fluid can be treated as a gas. In contrast water of course is 1000 kg / cubic meter.
I do not think interposing an amount of this liquid or gas would do very much to slow an incoming fist at any speed based on mass alone. It is just not very massive. I sort of feel bad for the demons if this heavy gas is their armor. Hopefully it smells good or something.
With edit, the liquid is now 1600 kg/m^3 or half again as dense as water. Water can stop bullets but it depends on how much water and how fast the bullet is going. I feel like I have seen someone (Mythbusters?) on the bottom of a pool watching bullets shot down into the pool.
I would not trust 3 inches of water to stop a bullet. But a slow moving, meter wide fist might get slowed down some. The meter wide (probably comparably tall) fist is a large 2d plane and it will need to push the water out of the way before hitting the target and it will expend its momentum doing so. Imagine you have a 3x3 meter foam fist on your hand and (in a science experiment) you try to your your associate who is lying under 3 inches of water in the kiddie pool. There will be a big splash as you displace the water and your punch will be slowed.
The difficulty calculating the problem is the magic. In the kiddie pool the water can splash up into the air which makes it easier to move. Completely underwater the water must move aside other water - more difficult. If the water barrier around your demon cannot displace air but is confined to the barrier then it would be harder to move. If the water were absolutely confined and could not leave its 3 inch layer then it would be very difficult to penetrate - but would transmit the force of the punch right thru, like hitting a steel unpadded helmet with a hammer.
Look how loosey goosey I was with metric and imperial up there. We Yanks are going to lose another spaceship if we keep up this nonsense.