I haven’t seen this unblockable before, but then again, I am new to this game, lol. Just throwing it out there.
C.HP, EX Headbutt, LP Ball xx MP Aegis, immediate HP, whiff MK Tackle pushing under Aegis, crossup Headbutt, …
The standing HP, Ball, and Aegis hit at the same time, and your opponent falls towards you. You may have to use MP Headbutt to get over them. Unfortunately, it seems like you have little time to perform the guardbreak (at least to me).
This combo seems pretty easy and does good damage, but it uses extra meter for the EX Headbutt. I haven’t tested it thoroughly. Try it on shotos or twins.