I'm not sure exactly as I don't do much Gmod editing myself now-a-days and all my techniques now are based around using round brushes and nothing else pretty much.
I'd take a look at Vman's more recent work with flashes in.
Rather than making them out of blood or spark brushes (which it looks like you're doing here, which I know is what Vman and I and probably Joazz used to do), try making the basic shape by simply using the lasso tool to select your shape and then painting it in with solid white colour. Then erase around the shape with some cloud brushes. It can be tricky to get the hang of but it yields much better results.
I initially paint a big white blob then erase around it with cloud brushes untill I get some sort of shape. Then add glow, throw in random spark effect, etc.
Hmmmm, I'll have to try this method you mentioned out at some point. But not in my next.