Something to add on to Jab 1 > F-smash is that even though it's only a true combo in training mode against Marth at 175, (my calculations say that it should work starting as 172) Marth (and most characters) have no way of escaping this at earlier percents. Airdodge starts on F3, so you should be guarenteed f-smash as early as 160. Add in rage, and you've got a viable kill setup.
Edit: Aerial Dolphin Slash has invincibility on F1, so Marth can get out once hitstun ends. So for Marth or anyone that has an F1 invincible move can escape if they're not in hitstun.
Edit 2: Jab at 175 vs Marth gives 36 frames of hitstun which is just enough to follow with an F-Smash. Assuming that f-smash is hitting on F13.
That's all assuming that I've got the right numbers here.
Edit 3: Figured I might as well post the numbers.
Jab 1 does 5%, with 30 base knockback and 55 knockback growth. 28 total frames, with the next action starting on frame 29. F-smash hits on frames 10-13, and because of the arc will connect on frame 13.
At 172 Jab 1 should give 36 frames of hitstun and 90.1121 units of knockback.
((((((172+5)/10+(((172+5)*5)/20))*(200/(90+100))*1.4)+18)*(55/100))+30)=90.1121
90.1121*.4=36 (actually 36.04484)
So on frame 5 Jab 1 connects, and enemy Marth takes 36 frames of hitstun and attacking Marth can act 23 frames later, which leaves exactly 13 frames for f-smash.
If any of this is wrong, please tell me.