Robin got buffed in the new patch (his down throw leads into anything except up b and down b) so now he has a combo that can kill at 50%. (I thank my friend Zelina56 for capturing the moment for me).
Down throw + Arcthunder (I’m using custom arcthunder but it works for default too (and its easier)), + down throw + forward smash (kills lighter characters without charging it, but holding it gives you a higher chance to kill)
If you miss the tech, then you’re dead especially if Robin charges his smash attack. If you grab the ledge, you’re safe and some characters like Jigglypuff can fly out of the Arcthunder.
Combo kills at 50% on most tournament legal stages (Smashville, FD, Town and City, Battlefield, Dream Land 64, Duck Hunt, Isle Delfino (specifically on the sections where the side of the screen will kill you faster), Lylat on the upper platforms as the bottom platforms will mess up the angle of the down throw therefore the opponent can get away even without pressing any buttons and Halberd during the moving section and when on the Halberd itself (but watch out for the ship attacking you during the combo. It can work to your advantage but don’t rely on it).
UPDATE: Things I should probably add to this. On lighter characters like Jigglypuff, this works at 30% but stops afterward due to her flying out. Same with someone like Peach where it works at 40% and stops working afterward. At 70%, this stops working due to the knockback of the down throw changing after 60% so you’re wasting your arc thunder if you try the combo at this percentage.
As a result, if you down throw, follow up with fair, up air or Thoron. At this point though, they should have taken more than 80% damage should they not die. Doing this at 0% will rack up 20-25%.
Reading the opponent after the second down throw is important. Not teching the floor, air dodging and DIing in or if the person does a bad get up option should they grab the ledge can get hit by forward smash. If someone tries to jump out of the combo, follow up with up air. If someone tries to DI away, Robin might not be able to hit them but fair should to make sure they don’t punish the combo. If someone tries to spot dodge or roll behind you, use down smash. Rolling away should make you use arc fire to prevent being punished.
Another thing to try is down throw into jab which is mostly guarantee up until 60%.