This is for completing 200 Combo Challenges. Most of these are not difficult however the last 1-3 can be hard.
There are 24 Characters without DLC. Which means there are a total of 240 challenges. This means you will need to average 9 per character. You will have some leeway as 24x9=216. If you can manage to complete level 10 for some characters that will also give you a lot more leniency towards how many need to be completed on average.