You should ideally not use the X-Gene Sub to rank up a character but to upgrade his/her gears, because you cannot use tickets for that and it takes around 400 X-Gene to upgrade all gears to 20 and be able to T2 a character. And most characters are not so much useful at T1 (especially not worth it to pay for them to have them only at T1)
Finish X-23's gears & put anything left toward ranking up her uniform.
At five stars, Weapon Hex can solo-clear SL floors up to around 10 (depending on the rest of her build). She can wait until you can drop a six-star RUT on her.