How long is xcom enemy within full#

What I did was sort of leapfrog my soldiers - the ones in front would go slower and kill the chrysalids that caught up, while the ones who were further back would use their full turn to advance. Your sniper cover will help with that, ideally, but eventually they have to move too. But if you go too slow the newly-spawning ones will overwhelm you, so it can be a difficult balance. The issue here is that chrysalids move quite a bit faster than you do, so you have to advance while simultaneously killing them. Once you trigger the air strike, you have to get back to the evac point.

A Support with the +3 move skill may work best. I used a MEC Soldier with the jet boots, but as long as you can get there and back quickly it isn't a big deal. Ideally you'd have someone close to the air strike trigger already, or at least able to get there quickly. From then on enemies will start spawning fairly quickly, so you need to move as fast as possible. Once you're set up as well as you can be, get into sight of the whale. Make sure they're far enough back to get out themselves, of course. Get a sniper or two into a good position to cover your people as they retreat, too. So just take your time and clear out all the enemies first.

Take it slow at first - before you reach the ship's hold, no new enemies spawn and you're not on any kind of time limit.