You have to get all three bridges down before your friends are routed or the aliens escape in order to win.

Turn on your ground grid.  The canyon walls have a tendency to surprise you.

If you don’t get the bridges down quickly enough, they will overwhelm your ground forces and you’ll lose.

Take out the bridges near your friends first, then seal off the aliens’ retreat.

Aim for the center of the bridges.  That’s their weak point. (You CAN lock-on to them.)