Jab to up tilt will kill around 125% on Demo characters and is pretty easy to setup. I'm not sure it's a guaranteed combo, but I believe it's too fast for anyone to react to. I believe the first jab has IASA frames that lets you cancel it into an up tilt, and you can use the jab start up to buffer into the up tilt.
Up smash might also work but it has a lot less forward range and is a bit slower. If they are right next to you they get pushed away on the ground level. They have to be at medium range in order for then to get popped up for the up tilt.
Jab 2 > S-Tilt / S-Smash 2
Boomerang > Spin Attack
S-Tilt > N-Air > D-Air Meteor
U-Tilt / U-Smash > U-Air
None of these are near guaranteed. The first one is not a setup at all. The second jab has way more lag than one jab, and one jab still doesn't work with f-tilt or f-smash.
Boomerang has too much knock back at the point spin attack will KO, and fair would be a better option after boomerang anyways.
Your third option is a string predicated on your opponent not fighting back or avoiding it all together, and ends with a really short range, hard to land meteor. I feel like you just made up a bunch of stuff you thought might work.