Boss: Ooka Blood, Blood Fencer, and Lord Onyx
Attacks: Mega Blade, Power Axle Omega, Impulse, EX Cross Flame
Blood Fencer's Info:
Attacks: High Blade, X Cutter
Lord Onyx's Info:
Attacks: High Blade, Impulse
For this achievement I mainly did Auto Play until one of his golems died then with the rest I used my best attacks and my EX attacks. This fight can take a while but you'll eventually be able to defeat him. Have Atsuma, Raigar, Yuki, and Lord Onyx in your party.