1. If I was with [enter name of Corps you want to exploit to get better] and found out you were just trying to feed yourself up the DCI ladder... You'd get absolutely no respect from me. I'm not alone in this thinking, I'm sure.
2. You're nowhere near good enough for corps yet, and you think you can easily get good enough to make phantom..? (!?!?). We had a rule at auditions: check your ego at the door. It's not just about chops. In fact, chops might even be in the minority. Attitude has a ton to do with it, how well you get along with people, attitude, ability to take what they tell you and adjust, attitude is factored in, marching ability/style, FOCUS/concentration, attitude ... get my point? Paul rennick will also ask you to sight read and solo on command. So yeah, have a nice day with your chops.
To answer your question though: practice until you think you can't POSSIBLY tick on it,, and then learn it off the left hand, then backwards, and then mark time in triplets. a.k.a. keep practicing