Difficulty rating explained:

1 - Easy, this is like genin and regular people... 
2 - Medium, you're stronger, but these guys may harm you, skilled genin and new chuunin.
3 - Equal, watch your step otherwise you might get seriously hurt, chuunin or new special jounin.
4 - Hard, better team up, this guy's tough, special jounin or jounin.
5 - S-rank, kages, skilled jounin, akatsuki... Yeah, if everyone idoesn't team up on this guy, you're fucked.