From your video, it looks like you blew the first explosive gel too late, so the Titan got to move forward a bit which allowed the first wave of guys to spawn early. You need to blow up the gel as soon as the Titan yells and starts preparing to charge so that he doesn't get to move forward at all.

But yeah, Fast Elevator sucks. The setup usually isn't too bad, but even if you do everything right you pretty much just have to hope that while you're pulling the wall down whoever attacks you has a slow attack animation. That trick is one of the many reasons I stopped running NMS.