Practice toe 3s without a hanlde pass that will get you used the feeling of the blind landing and get used to the boat tugging on you in that position. I would suggest practicing the rotation on a tramp if possible.
For most people doing an OA TS 7 is easier than a on axis 7s. I have noticed over the years teaching people this trick, that the hardest part for most people is do make the trick one motion instead of 2. Most people, when learning think, do a 5 with another 180, that works for gpoing from 3 to 5, but not from 5 to 7. That causes them to spot the landing at 540 which stops your rotatoin dead and will cause you to get owned. Learning on a tramp will help you make the move 1 motion.
Remember Getting Slammed is part of the game especially when you are trying to up your game like this. The First 7s you try will result in some pretty good slams, but in the end the fist time you ride it out is amazing.
Good Luck hope this helps-