When cut scenes occur dealing with who you need to assassinate, you will notice a little "glitch" type thing go across the whole screen, at this time push any button and it will change the point of view on the cutscene. This will occur for every assassination so pay attention to the cutscenes and you will get this before game end. You can miss a few but to be safe always pay attention and look out for the glitch as several occur during each scene.
Note: You CAN replay Memory Blocks of prior assassinations and the glitches will still count towards the "85%" even if you've gotten them before. So if you don't get this by end game, simply replay some assassinations and it should pop up eventually.jrath05
suggests replaying Chapter 1, although there is no assassination, there are many glitches to count towards your 85%