No matter how many times I pinch, polish and so on, these type of edges always cave in. When i pinch one side, one of the others will sort of “drag” towards it, smoothing the corner like on the picture.
Anyone has any idea of method or maybe some kind of plugin that helps with sculpting crap like this? It’s very annoying and my hand hurts already from all the methods I tried making this corner “hard”.
Could try scrape brush with maybe view align. Anchored stroke could give a bit more control to it too.
Another result when hiding already flat faces
That doesnt work - not enough control, for instance it will do nothing but mess up an edge like this:
Doesn’t Pinch/Magnify brush do just that?
It will be better to show the whole model so people can see how to help. Blender dyntopo is not really idea for hard surface but here is a short tutorial by Ben Simonds http://bensimonds.com/2013/05/01/smarter-scrape-brush/ and you can check masterxeon https://www.youtube.com/watch?v=hoaPM1C1vAE
Hope this help 
Another thing that might work is to use the draw brush with the curve set to really sharp- instead of dragging in the sides like a pinch brush, it’ll just raise the area directly under the cursor to a point. With stroke smoothing and a careful hand, you can get most corners into a good shape that way.
Well, the flatten brush.
There is a lock to the left of the “area or view plane” Area is probably the best.
See what it’s doing. Avoid overdoing it.
