Hopefully this is the last Community Challenge for this game and they never add more.
One way to get a easy S rank is to do Mission Mode 49 which you unlock by completing the second chapter of the story.
This was discussed during the previous solo challenge:
Another spot you can try if for whatever your not that far into the game is on the very first chapter after a few of the mission segments you'll meet up with Grimlock (he's the dinosaur robot) and you'll have to take down a door that has these arms with buzz saws on them that you need to dodge in order to take the door out.
In this tiny area you'll be confronted by two normal enemies, beat them up. Next two flying enemies will come in on the building, simply shoot them. Should take two shots each.
Then you'll have three more regular enemies to beat up.
After this killing these enemies this mission segment will end and it will show you your rank. If you did it in under thirty seconds you should get a S rank.
Feel free to press start and reload the checkpoint if you want to grind for S ranks on this segment but I don't really think that will be necessary but do it a couple times just to make sure it registers that you did at least one.
You don't have to use the two specific spots mentioned above as it doesn't matter where or how you get a S rank, you just need to get one and you'll be good.
The two listed above are just quick and easy suggestions.
The challenge should unlock for everyone a couple days or so after the challenge time expires if the goal is met.