Its really easy with some practice...
Do B+K, then immediately MASH 611111A+B, just press 61111111 and in the same time A+B A+B A+B...the last part is easy again. Just press 44B+K or 66B+K (both work, you you switch the directions) as soon as the second hit has connected. Or if you go for the wallsplat or RO keep mashing A+B...you dont have to input 61B at all.
When you do it after ch 4A you have to actually time the second hit a bit better, but try to do it without ch 4A first.
The mashing workes really well for me.