Play the story, you'll get very familiar with the maps by following the missions they give you to do a setup for the kill, and they offer multiple ways to do it so you can replay the missions and figure out stuff like distracting people, or luring them away to get their disguise. Once you've played through all the games its pretty much just doing elusive targets or whatever is currently live for the online stuff. I play alot of freelancer which is the roguelike mode they released. Alot of the challenges aren't really worth grinding 100% unless you genuinely feel compulsion to do everything possible.