Shuma’s infinite is all about timing and positioning.
Timing: the main parts that really require strict timing is during the rejump after the FS to get the infinite started and linking j.mk > j.lp.
The linking j.mk > j.lp uses shuma’s normal jump glitch which allows him to do another normal afterwards. The timing is after shuma stop’s spinning, he is recovered enough to do another attack. His spin animation is the ‘pause’ you need in between hits. To practice the timing on this, do j.lk, j.lk on a standing opponent. Then, even if the j.mk whiffs or hits, do a j.lp as soon as possible. Then it’s simply j.lp, j.U+fp with a nano-second pause in between.
On a side note, i do a slight variation of the infinite. s.lk, s.lk, sj.lk, u+fp, u+fp, land, [j.lk, j.lk, pause, j.lp, j.u+p] I like the j.lk because it’s much easier to time and because you light the opponent fall a bit more making the rejumps afterwards at lot easier.
On positioning: with this infinite, you don’t want to be exactly next to the opponent. You want be be slightly away. The perfect positioning is to start off with a j.lk, j.lk first, then when you land, that’s the distance you should be away from your opponent. When you launch the opponent, you’ll want to sj. straight up, not u/f. You’ll notice that sj.lp will whiff because of the space, but sj.lk will hit perfectly. Another thing is, if you’re too close, the opponent gets hit by the middle part of the j.u+fp. This causes a couple problems: a) it will usually whiff, b) if it does combo in, you are too high up so the rejump wont combo in. So, if you are at the correct distance, only the tip of the j.u+fp will hit the opponent, giving you enough time to land and repeat.
Also, as far as practicing goes I find it very easy to do on magneto.
