More than once I had a good team in defense, had one single battle, and got E-ranked for 2-3 days until more people attacked me and I went up to rank B or C. The other way around also has happened.
To me it seens kinda unfair these situations. To grade a defense team A or E based in one single attack.
So I suggest grades would remain C until a 3rd defense is made. Then you have more data to grade that team.