Do you need a really high trust level? Or is it pretty easy to get as long as you're somewhat competent at the missions and don't act like a jerk?
For the missable scene? I've always got it when I've played good with her, chosen the right answers, not done anything to lower trust like hit her with friendly fire, played well to earn trust in combat etc. I tried it a bunch of different ways but according to other people they've had varying trust levels and still got the scene, so I think it may be to do with how much you have her in your squad (without pissing her off) or something.
I'd suggest using her frequently and choosing the right dialog choices, avoiding friendly fire etc, it's not that difficult to get in my experience.
Just have faye in team for every mission and dun say the wrong things. Also lead the way in combat, successful headshots and awesomeness will add teammate trust.
I only had like 2 bars for her trust until the romance scene then it became 4 bars.
Well actually its five full bars now after the KISSING scene...