Here is a link to PowerPyx’s Challenge Guide which gives details on how to get 3 Medals for every Ranked Challenge.
For the Combat Challenges, I recommend trying to get a 5x Combo as quickly as possible, mostly by and then over and an enemy until you get it. After that, press + for the Bat Swarm combo. What this does it increases your combo by 2-4 hits, and stuns everyone in the vicinity. This way, you don’t really have to worry about getting hit for almost every Challenge since you’ll have more than enough points by the end of Rounds 2 or 3.
For Predator Challenges, patience and luck are essential. While some may want to get a fast time for rankings, they aren’t required for getting all 3 Medals.