There are only two things that gave me trouble in the challenges. The first was the combat arenas that require 60000 points for three medals. You need a long combo in at least one or two rounds. When I first played them I wasn't good enough to hold it consistently. Probably because those have titans or a one-armed guy that can mess you up.
The second huge struggle was that final predator campaign. There's a negative perk for booby-trapped vantage points, but the first two require inverted takedowns. So you have to save it for the final challenge, and you've only got three retries.