If you are going to smash the fish against a wall you are better off putting it in a bag and hitting it with something ie a baseball bat, golf club (if you don't care about the club) or something like that. Smashing it against a wall will have a lot of variables, what if the fish hits the wall length wise? There is a lot more body for the force to spread out on, reducing the force per square inch. If you use something with a much smaller surface area, like the bat, you are getting the maximum force you can. Then you can chose where to apply it.
Using this approach the best way would be to smash it in the head with a baseball bat, much more accurate, a lot more speed and force.
All that said, I would absolutely not recommend this. You could miss, or hit it in the body and not kill it but injure it. Same with smashing against a wall, what if you don't get it on the first shot, then you are just torturing the poor fish.
I have ended plenty of fish lives through the years fishing. Fastest, easiest and most accurate way, cut the damn head off and be done with it. If you don't want to do that than I would absolutely go with the ice water bath. My suggestion for this is halfway fill a container, just large enough to house the fish, with water. Dump in as much ice as you can fit, put said container in the freezer for like 20-30 minutes. This will will the temp way way down and should not be long enough to actually freeze all the water. Take the container out, if the top is frozen over break it up with a fork or something and drop the fish in. I bet you it doesn't last 10 seconds. Even if it lasted 10 seconds or longer it would not feel a thing because its system would go into shock.
The same idea as a humane falling into ice cold water like off a boat in the ocean or whatever. You don't last long, and your system literally shuts down before you even know whats going on.
So all that said, I would not just smash it against a wall. Either cut the head off or drop it in freezing water if you wont go get clove oil...
my $0.02...