OK, here seem to be the unlock conditions (warning: DO NOT EXCEED THEM! I ran into problems)
Set Rounds to 1
Super/Ultra Combo Finish on Seth (Source: TA)
OK, now for the fun part: Here are my tips:
1. Second controller mandatory. You are going to screw up unless you're someone like Daigo Umehara. New Challenger when you screw up.
2. If you have a fight stick and know how to use it, I'd highly suggest using it on this one.
3. If you're not a pro, Cheap 'Gief with the Lariats is the best way to deal with this problem. Use jump buffers to land 720's, I'd suggest going to Training Mode to get the timing down and only attempting 720s when the opponent gets stunned.
4. If you don't have a stick or a MadCatz Fighting Game Pad and have to use the PS3/PS4 controller, here's the controller setup I use:
Square: Light Punch (in case you ever need a SPD)
Triangle: Your choice
X: LP + MP + HP (for easy Lariats)
Circle: HK (you will need it to stop cheap Ultras)
L1/R1: Your Choice
L2/R2: TURN OFF! This was by far the most frustrating thing for me trying this with those buttons on. Once I turned them off this was far easier. And yes, I'm probably a cheap-ass.
Considering Capcom is giving everybody the big FU in terms of completing their games and making them near impossible, enjoy people.