The most maxed out combo I can think of off the top of my head for Ermac has several rule stretches/breaks.
HK, LP, TKS, RH, TP, TKS, otgHK, LP, JK, aaLP, fireball is 57%.
TP to the TKS can be difficul to get unblockable because it's only 2 hits inbetween, and the JK, aaHP, fireball is also tough.
A more practical combo would be:
HK, LP, RH, TP, TKS, otgHK, LP, JK, HK for 53%
JPS, HK, LP, TP whiff, aaRH, TKS, otg, HK, LP, JK, aaHP, fireball is 60%.
You might be able to get a JK at the end vs say Jax instead of a fireball.