General

Killing the 2 shoulders and the core are the only things that decrease the difficulty. Killing the legs does nothing. However, I'd recommend against trying to kill the head without killing anything else. That gives the head far too much hp.

you start off by going left or right and then up the hands and then from when you go into the arm you can either go up to the shoulder or into the central hub area so skip his should, head there and then go to the head.

Sometimes the monsters drop a teleport rock. There's 3 kind of those rocks. One teleports you to Right shoulder, Another to Left shoulder and lastly straight to head. You just gotta be lucky to get one to drop. Its not that hard drop rate though. But, I believe destroying the legs won't change the level difficulty. Only Shoulders and Abs would.

I don't think killing the legs affects the difficulty.

You can use teleport rocks to reach the left/right shoulder and head (they drop infrequently from the monsters inside), but I don't think there's another way to get to the abdomen besides killing a shoulder and accessing the core, or going up the legs.