If you are trying to complete this week's challenge, one of the things you will need to do is turn a few friends into enemies. You can easily do this by first making friends and then doing hostile actions towards them every year. However, you will run into problems early on as chances are they will unfriend you rather than declare you an enemy.
Because of this, the best way to get around this problem is to simply make a ton of friends and accept every friend request you get in the game. We also recommend doing this while your character is in school as you will have access to the classmate list, which can be used to make friends.
Just view the classmate list and then boost relations with your classmates by paying them compliments and other nice actions. This, combined with accepting every friend request, will get you a got a sizeable friend list. Once you got enough friends, procced to insult your friends multiple or just once every year. It will take time as they will more often than not just unfriend you, so the trick is to make a ton of friends and insult them until you get the right result.
Do this, and sooner or later, you will have a few friends declaring you an enemy. However, make sure to target at least three female friends, as it is required to complete the objective in this week's challenge.