The piston fist whatever weapon deals damage through enemy armor. You should be able to buy one from a checkpoint. There's one that goes into weapon slot 1 and uses energy and the other one goes to weapon slot 2 and uses some ammo. The second one causes an explosion that knocks you backwards after hitting the enemy which can be nice in some situations but it's not recommended for farming. As for how to hit the rockmonster, fly around a bit and see his attack pattern, then find a suitable timing to hit it. If you can't kill the rockmonster with 2-3 hits then you might want to grind some other places for ores to increase attack or to sell for money so that you can upgrade yourself.
As for the girls that can be found being raped around the world, I'm not sure if they are so relevant. They seem to be a sort of side quest where you save them and bring them to a teleporter and they give you a reward. There is one character you can save in the jungle from a boss monster but that one's a male I think.
Btw I'm lost in the volcano. I've found some nice loots and stuff but I don't know which way to go to progress. Someone posted the full map here on another page so I can see that I'm supposed to go left somehow but no route seems to take me there. Do I go left or right from the teleporter in the middle of the volcano area and then do I go up or down?
EDIT: Ok, I checked a complete save file and backtracked. There's a place where some bandits are fighting a huge orange firebreathing lizard. The cave above that fight seems to take forwards.
Is there a way to lower the respawn cost?