I'm going to assume that you've never tried Ramuh Ex cow since from my experience it was the hardest out of all the extremes in terms of party coordination and ADD phase DPS checks. And from what you said, Ultima HM and Levi Ex shouldn't be counted as progression either o.o;

Savage Coil is progression no matter what people think of it as; there should not even be any doubts... Despite its small reward of granting you a bigger e-peen it is still currently the hardest content available.