In this shot I had to get rid of the greenscreen and add sky as the background since the city is in the air at this point. I added the CG Sub-Ultron and added the CG shield from Captain America. I added lights on Black Widow's uniform and weapon. Added background, midground and foreground layers of blowing dust and flying debris.
Here's another before and after. Added CG shield and CG Sub-Ultron. Added additional burn marks on the rear building. I had to remove trees that were in the background, to give the impression that the city is up in the air. I removed equipment from the lamp post. I also replaced the sky.
For this shot I had to do a lot of cleanup first to get rid of big cranes and lots of crew and production equipment. I had to rebuild the bus stops and most of the playground. I added the flying Sub-Ultrons their thrusters and the shooting lasers. I also added the explosions.
This is a shot right after Thor throws a bunch of Sub-Ultrons against this truck. I added the Sub-Ultrons and removed some wires that were used for the real explosion that happens in the shot.