What was your set up for 550k, I feel like i should know this.
(2A), 2B, 2C, 6C, 5A, 5B, 5C, 6C, 5A, 5B, 5C, S, j.B, j.B, release level 3 buster, L raikousen (hold L), S, j.B, j.B, j.C, j.S, call assist, H Sentsuizan, release level 3 buster, H Ryuenjin
You need to charge the buster while doing the loop at the beginning of the combo, I tend to alternate between A and C, if you just practise eventually you'll find the method for charging that's comfortable for you. Also note that in the 3rd repetition of the loop, there is no 6C. I found that if you hit with an assist or an aerial move at the start of the combo, hit stun deterioration kicks in at the end of the combo causing the j.C to not connect, which is why I don't do the 6C.
Also, the last bit of the combo only works in the corner, because otherwise the j.S knocks them too far away to OTG with Sentsuizan. However, on most characters, the combo will always get them into the corner for the assist + sentsuizan setup.
There are many different assists that can be used. I've personally used Dante's Jam Session, Phoenix's TK Shot, Ammy's Cold Star, Taskmaster's Horizontal Arrows and Akuma's Tatsu to success. Depending on the assist, you have to time the Sentuizan so that it OTGs just as the assist hits. Then after that, depending on the assist, you may have to jump and then release the buster, then do the L teleport into H Ryuenjin (or Rekkoha for more damage, but then it wouldn't be meterless). Otherwise you can just release the buster while standing and then Ryuenjin/Rekkoha.
You may also replace the Ryuenjin at the end with 5A as a reset. When they recover in the air you can go for an air throw and then combo after that, a j.A to beat an attempt to tech the throw and then do the air loop, or a standing L teleport to go behind them and then just do the same combo at the top of this post.