Hey Zuzu, what was the notation for that Mr Karate combo you were tellin' me about the other day? I know what it looks like, and it looks really simple...
It is easily the hardest/most annoying part of Karate. I have no problems performing this relatively close to the corner after one rep but midscreen is annoying. lol
st.C, f+B [HD] st.C, f+B, [qcf+A (HDC) dp+C~A+B, hcf,uf+B] x4, qcb,hcf+AC
In the corner, you can substitute tiger knee Hien (hcf+B) for his A version of his FB. It does make the combo easier to time. If you do decide to go with tiger knee Hien, from my experience the timing is more lenient in the corner than it is in midscreen.
I like doing tiger knee Hien 'cause it looks cooler but it has messed up my timing for the midscreen HD combo.

You can try practicing the drive cancel first before doing the full HD combo.
st.C, f+B xx qcf+A (DC) dp+C~A+B, hcf,uf+B, qcf+A (DC)...
I practised this extensively before practicing the HD combo. The thing with me is that if I land the qcf+A after the tiger knee Hien during HD mode, I can get the full HD combo starting from the corner, no problem. Holding down the A input in my qcf+A helps sometimes.
Sadly, I could do the midscreen HD combo 7/10 times before I slacked off and started doing random shit in training mode.

All you need to do is practise more.

Edit: Alternatively, if you don't mind losing out a bit of damage, omitting the f+B after the HD activation makes the combo so much easier. It does 6 less points of damage starting from st.C and finishing it with NM.
Also, it seems like MADKoF13's youtube channel is starting to upload videos after 2 months and so weeks of inactivity.