You shouldn't really be here, since you're below 13 you're violating the COPPA.
and seems like flash isn't desighed to do it that way( or is it?)
Why shouldn't it? There are a lot of great frame by frame Flash movies, ranging from stick fights to Adam Phillips' movies like Waterlollies. Note that he used to work for Disney until he resigned.
Tweening is just there to ease animation. I has far less variety in frames, but it saves time.
And don't let age as an excuse. Unless I got my numbers mixed, JAZZA startedhis Paladin series, with the first episode submitted while he was just 13. The last episode was started when he was ust 16 and finished it 2 years later.