I managed to get the Giants Fist Achievement without even trying. I was playing the Colluseum fight against Lightning and Amodar and managed to earn the Achievement just from fighting them. I haven't acquired any ultimate weapons. I just raised the stagger meter to over 900% and I think it was my Behemoth, whose been my main Commando for a long time now, who scored the 99,999 blow.

So if you want to accidently get it, a maxed out Behemoth seems capable.