It's not very difficult if you do drag soldering rather than attempt to solder the pins one by one. I do it in a slightly different way, just put a ton of solder and clean the shorting with braid.
I'm not so proud of my soldering skills, while still managed to hand solder more than a dozen of such FPC connectors (0.5 mm, 45 pin, Wurth Elektronik 687145149022) with just a standard Weller station for a project at work, and only non-fatally screwed up my second one.
Yes, drag soldering was the only technique I considered. However, it didn't work very good. As said before, the main problem was that solder created shorts between pins, and some others were left with bad connections. I also tried to use braid to remove the excess solder, etc., but I felt the more I tried to fix it, the worse the result.
I once watched this video about SMT soldering, and while the guy seems to be using just a soldering iron and flux, the result is outstanding. I don't know however which specific equipment and solder/flux he is using, and if it's real or there's some kind of trick involved :-D https://youtu.be/5uiroWBkdFY?t=112
Maybe you can try some different brands of braid and flux.
However, actually I got four out of the dozen done without even brand braid, but with some scavenged copper grounding braid, as the proper one usually on the bench was stolen by a colleague :).
I'm not so proud of my soldering skills, while still managed to hand solder more than a dozen of such FPC connectors (0.5 mm, 45 pin, Wurth Elektronik 687145149022) with just a standard Weller station for a project at work, and only non-fatally screwed up my second one.