If you mean this combo:
http://8wayrun.com/threads/natsu-1-02-combo-discussion.8868/page-6#post-490388
My advice would be don't do it unless you have perfected it.
Even though HRD is wrong (sorry HRD) and it cannot be teched out of if done right.
It's just way too hard to be practical.
Plus when the UB Bomb (It only works with


as far as i know) hits while the opponent is too close to you, the first, early CE required for the combo to work can miss completely. So you waste meter and quaranteed damage.
I say go for Bomb~ CE~ PO Air Throw
Or Bomb~ CE~ PORC~ A+B~ CE (no relaunch) although here the opponent could (maybe, not sure on that one) tech to avoid the last hit of the second CE.
Unless of course you can do the Double CE Air Throw Combo reliably
(i hit it maybe once in 20-30 tries in practice mode)
and are able to confirm at which range you hit with the UB Bomb.
Even though it could still be character and off axis dependent.