How do you make organic shapes?

hold shift in db2, close your eyes, move your mouse around the map and click LMB now and then

congratulations, you have a non-orthogonal rock

Start with a spiky basic outline of the rock formation you want to draw. Then, come back and add vertices between the first set to add roundness. Tweak it until it looks natural-ish.

Well, I like draw weird lines , but not that much, because texture alignments become painful process. Here, example of my stuff,this place is very messy, I just quickly textured for you.

 

P.s I'll add map plan here too. 

 

Second option on the left bar of GZDoom Builder is "draw curve." That might help, if you want smooth instead of jaggy.