I have recently won a competition with a terminator fig similar to this.
Basically, you have to cut the body at a diagonal so 1 leg pin can still fit in. Doing at a diagonal means it does not effect the neck pin, so don't worry about that for now. For the legs, you want to just take the right leg off but leave the hip bone.
Then cut the skeleton up and glue 1 leg where you took the other one off and cut the skeleton body at a diagonal so it just overlaps and fits into the normal body. Then just glue it all together and paint it.
For a skeleton more like in the picture, instead of cutting at a diagonal, just cut straight down the middle. As you do this you should be careful to cut around the neck pin like in the picture so the head can still attach. (This should hopefully still leave the leg pin connector compared to the diagonal one.)
I managed to cut myself ALOT doing this fig and eventually used a hacksaw to cut through it. So if you are going to attempt it use a good sharp knife and cut in a sawing motion. Failing that, carefully use a hacksaw. (be careful not to ruin the fig!)
If you want anymore help or pictures on how to do it, you're welcome to pm me!
Once you dissect the photo it's relatively easy to figure out what I did. I made a bunch of these 'undead' something like 8 yrs ago and decided to use one of them as a product minniquin. The skeleton body actually fits inside a minifig torso (in this case a really, really old torso) so I didn't have to cut the rib area at all.