For Ness it's pretty easy to combo with PK Fire and Back-Throw for ultimatum damage. First PK Fire your opponent. They should be trapped in a fire pillar. Rush-grab them, giving them minimal escape range, and do whatever inputs you want! (I always pummel a few times and do a back-throw, safest.) This is really easy to do on CPU's rather than a lot of human players, so keep in mind what you want to do for this.

For Lucas, it gets harder. PK Fire doesn't trap them in fire -- it sends them back instead. To make up for this, Lucas has a tether-grab. While your opponent is stunned (if you're facing high-level experts, a very short time) or when they get back up (safest option) you can grab them and do whatever you like (Down-throw is the best option for me).

By the way since you can't put Smash 4 in general for these guides I just put Smash Wii U since it's the most popular out of the two.