I'd use forms to cast the dam, like a small kerb, then pour relatively
wet crete and let it find its own level with minimum striking.

Yours is a good idea. Now, I just need to find the right material to
build the form and prevent the thinly poured concrete from leaking
out. Then, I don't need to worry about stiffening concrete and
entending work time.

Be sure you use a concrete that will stand up to use in a thin
pour - I'm not certain, but I think you should use one of the
vinyl reinforced or surfacing concretes.

I probably will use leveling compound. They hold up quite well in thin
pour based on my experience in my past failed attempt to level the
floor. When I tried to knocked out the high spots that I mistakenly
created, I have a hard time to knock it out if the high spots were
make from leveling compound. The down side is that leveling compound
is quite expensive. But I should not need that much if I only use it
to form the dam around the wall.

Thanks.

Jay Chan