I sometime run in to this. The bad bad boys are the sear bar plunger disconnector pin (small as a push pin in diameter), the front sight blad, the coupling link pin, and the small pin that goes through the top of the rear toggle link that retains the middle toggle axle pin. Soak in Kroil 3-4 days, then put in a freezer in a zip lock to keep oil from getting in the freezer, leave it over night, then add some heat from a blow driver. Hopfully the metal around the offending part will be slightly larger (due to the temperature diffential) than than the pin and the pin can now be removed. Notice this takes a long time and a lot of patience. I just removed a Navy front sight blade that had been through Hurricane Katrinas salt water and the blade was damaged so I knew it would be tough, 3 days in Kroil and it popped out, YUREEKA!
PS sometimes I use a lead brickette (from handgun bullet casting operations) to hold the parts during the hammer/punch operation