Having just worked on mine to do some tightening of the chest armor i can describe how to get it off and to get to the chest.
The simplest way with the armor is to pop out the underarm straps, those pieces of plastic which are part of the front of the armor peg into the back or the shoulderblades of the armor. Slight pulling and prying with careful tension can pop them out of the shoulder blades, once both sides are popped off they are flexible enough to bend out from under his arms and then pull the armor off (with the head already off).
Once thats done you have the figure without the chest/shoulder armor on it, just roll the very large open collar down over the shoulders, the suits very flexible and is preshaped so it will go back to its original form even if its been rolled down over the body. BUT note that the sleeves are glued to the elbows to keep them in their proper place, so dont go to far with the sleeves, i just rolled the suit down to his abs, i did it with rolling the suit down and outward so the inside of the suit was exposed, so that any debris or mistakes wouldnt hurt the outer suit.
Then you do what you want with slimming, dremelling the pecs etc. I ended up trimming the under arm straps to be shorter, and then reglueing them to tighten the chest armor, because fact is even if you slim the body the chest armor is already preset to a certain size so you have to tighten it, so i just cut some size off the ends of the plastic straps and then sized and got it how iw anted then put the armor back on and carefully glued them on. It's not easy but its the only way i know, since its what i just did.