The Scarmiglione can be described as a large Knight-like BOW with a sort of shield on one arm and a lance/sword on the other. It has the ability to split apart depending on how you damage it; whether you attack it's legs or head/torso. To get the achievement, you will want to use a high damage weapon, such as a rifle or magnum, and go for rapid headshots. In general, weapons that use many bullets (or pellets) to kill the enemy, seem to cause the transformation faster. One thing to be aware of is that they usually move toward you with their shield over the area you're aiming at, so mix them up by aiming at their legs, then quickly shooting their head a couple of times and repeating the process. They are also known to be very vulnerable to shock grenades, which can help buy you some time and keep them in one spot so you can unload on them.
The best method for this is to pick up the rocket launcher from the hold in Episode 7 or 8 (during the race to activating the UAV is the easiest) and keep it handy until you reach the lab with Chris in Episode 8. Once you enter the decontamination area you'll get attacked by the first Scarmiglione. Aim at his feet with the rocket launcher so he moves his shield down there, then quickly aim at his face and pull the trigger for an easy win (I'm actually pretty sure I missed him, but still got him no problem).