I found that learning to breathe is essential for a 100 contact run. You need bail tricks that take such little effort that you can gain a second wind within the run when need be, even if you repeat the bail a few times in a row, like, 5 infinities or something to regain control of the bag, your muscles, and your air supply. I think what I did was just bail runs of 100 and then slowly added in a few non-bails at a time. Try to do, like, 20 osis > 20 butterflies > 20 ducking clippers, and keep repeating, or something, or even 40 of each. Another way is to do bail drills on repeat, like, butterfly > pdx mirage [repeat]. And for that I find that it helps to not count contacts, but to think of each drill rotation as a single contact. So that's only 25 contacts of the drill you need to do to reach 100. Once you can do 100 bails in a row start to do the same, but add a few non-bail (but still easier) tricks every 10 or 20 contacts-ish.
Basically, if you can't do 100 bails, you won't be able to do anything harder. And even just tons of bails is great for stamina. I do this to warm down almost every session. Not necessarily 100 contacts, but, like, 60+ish with some kind of bail repeat or drill..