What species of fish is it? Dropsy is usually a symptom of something else (often a bacterial infection) in which case an anti-bacterial medication such as Esha 2000 will give the fish more chance of recovery.
Cement is strong enough if you apply enough of it and know the physics. The wall will also have to be strong enough.
Also if you use cement, understand that cement needs to be "cured" before adding fish or the fish will die!!!!!. Google "Curing Cement" for more details. Why? ---- Simple Explanation: Cement when contact with water will "leak cement" which makes the pH rocket to a toxic level for most aquatic creatures. After in contact with water for a while (4-6 weeks), they got cured and don't do this anymore. But then you need to completely change your water in the pond completely.
Alternatively.... I've never thought about doing this outdoor.... but you can use foam!!!!! Yes! Foam is lighter and comes with many color, you can even paint them. You can attach small rocks to them or woods. Or use dried and crush coconut shell to make it looks natural. Google "Paludarium" or something like that, I forgot I it spell. Most of the time people use foam to build rocky walls that look very realistic but pretty light!!!
And like others have said.... we really need visual image to understand what you said best :)
Drawing then if no picture, i really am trying to help. Cement does not need to be in the pond, water touching cement then running into the pond can also kill the fish, that's why you don't often see any "brick" fountains in ponds. I am also wondering how the water will flow? I am guessing you want it to flow along this slab? (It might do, but will probably run off the sides) and what is this slab made from? Like I said a drawing really would be appreciated, doesn't have to be a work of art, draw it on paper, take a picture and post that. (its what i have done)
I did know in the end. Breathing got more rapid. It was then confirmed by his very slow painful looking movement when I tried to net him.
Don’t like how it worked so slowly even though I know it’s the kindest way. After a minute he went upside down and was breathing like that upside down for about half an hour. Before finally going on his side as he died.