I position my troops on the side of the wingtips (out of line of sight, so the aliens have to cross 2 corners to get LOS on my troops)
Position one troop outside at the side near the front, and use googles to look into the ship.
Once I have gathered enough strength, I usually run one GL up, snap shot, run back.
If I think it's clear, I run one assault, to FAR away, (behind cover if possible) with line of sight, to the side of the exit. And put him on overwatch. (possibly send a medic as well). This guys job is just to draw fire (and live... hopefully)
If there are aliens there, they will jump out and engage the guy at long range.
At which point I resume grenade launchers running out, snap shot, run back
After a while, you will reach a breakpoint, where you have enough troops (and they have too few) and you can just rush in from the wingtips (note, you WILL need close combats to do this, as you consume close to 20 TU's getting from the wings to the doors)