Basically I am a lowly rank 819. Started playing 2 months ago and have become addicted. My question is, if I am able to beat a rank 900 in training 3 out 10 times, should I make him harder or keep playing him until I can consistently beat him every time?

Well, you can no doubt learn by playing the same personality until you consistently win; unfortunately, that will only prove that you can consistently win by playing that one personality.

Might I suggest that you try the different personalities suggested by the Post-Game Analysis? That way you can get stronger while playing against others with different styles of play.