Follow the onos.
So I have seen a few people complaining how skulks don't fare well end game. Its true, 1v1 end game fights really screw the skulks. But do you see that big 'ole onos over there? He's big and scary and has lots of health. I bet the marines are gonna shoot at him. Stay a few steps behind to avoid death by grenades aim at the onos. Once the onos has closed the gap and is in the marine's faces, make your move. The marines probably wont see you until you have you jaws around their ankles.
Smart play can compensate well for a "weak" life form.
Just my two cents. I'm sure other good options exist.