1. make up, masks etc
2. modelling - lots of films make small-scale models and edit it in with real life stuff if they can't make something happen on real scale. e.g. on The Dark Knight, in the scene where the Joker is shooting at cars with a bazooka, there is one shot in which the Batmobile (Tumbler) is actually a model because they couldn't make it jump over a car in real life inside the tunnel without destroying the public tunnel. They mixed this in with the rest of the full-scale footage
3. We include GLIMPSES of Nemesis, making it edgier and scarier whilst making it easier for us because of less work. For this we can use puppets like psuedo-animatronics - imagine Nemesis' face on a stick/wire poking through the darkness to create the impression that he's there, but that's the only time we see him. To get a feel of Nemesis, we could have Nemesis POV shots to go with this, done in hand-held camera watching the terrified actors flee in front of us.
4. We don't do it.