Unsure whether her combos are universal for the entire cast but I am having trouble landing b12 after jk~air sai. Is there a trick to timing it? Do I have to run for it to connect? Just need to understand so I can optimize my combos w her. I have been practicing on sub zero
Yes a quick run cancel before bringing out b1,2 is needed most of the time for Mileena's midscreen jk -- telekick -- air sai Piercing combos to work. However, two things to keep in mind when doing Mileena's jump kick into tele-kick into air sai combos:
One, how deep a jump kick is will dictate how fast you'll hit the ground and recover in order to do a b1,2 or b2,2 (for ethereal and ravenous) extension. For example, you roll and then immediately j3/4 the opponent at the peek of them being juggled by roll, you'll likely be in the air longer and wont have enough time for a b1,2 or b2,2 extension. This is especially true for b1,2 because its active frames were nerfed meaning the attack's hitbox wont stay out as long. So to remedy this, after a roll what you would do is hit the opponent with j3/4 lower to the ground. The lower to the ground you hit the opponent with j3/4 the more time you'll have to recover so you can extend with b1,2 or b2,2. You still want to input the tele-kick after the jump kick immediately though, this will keep the opponent from falling too low.
Secondly, how you time your air sai dictates how fast you recover too. You want to input air sai immediately after the tele-kick. By doing this your decreasing your recovery time. If you were to delay the air sai, it increases your recovery time and distances you from the opponent, but also it adjusts the height of the opponent lower. A lot of Mileena's corner combos benefit from adjusting the height of the opponent lower by inputting her air sai late. But in the case for midscreen Piercing, which I assume your asking for, you would want to immediately input air sai after tele-kick so your b1,2 will hit.
But some strings into roll create different spacings with Mileena and the opponent. I don't think this effects too much other than what jump kick you should use. And of course it would help if you immediately run cancel once you've hit the ground from air sai so your b1,2 or b2,2 will hit.
TLDR;
1) delay your jump kicks a little
2) immediately input tele-kick
3) immediately input air sai
4) a quick run cancel immediately after you land from air sai
5) immediately input b1,2 after that quick run cancel