Party- Fang, Lightning, Hope
Paradigms- Guerilla (SAB, RAV, SYN)
Relentless Assault (COM, RAV, RAV)
Perpetual Magic (RAV, MED, MED)
I had Fang's Commando role completely maxed out when doing this (post-Chapter 13). In the Archylte Steppe, find an Adamanchelid (one of the large gray monsters whose legs you DON'T have the option of attacking). Auto-Debuff with Fang while Hope casts Haste.
When everyone has haste, switch to Relentless Assault. After filling the chain gauge about half way, call out Fang's Eidolon, Bahamut, while staying in relentless assault. Press the X button when the Gestalt meter is full and then press A for the remainder of the battle. Bahamut's final attack did 411k+ damage for me, killing the enemy and popping the achievement.
If you are underdeveloped and in danger, switch to Perpetual Magic to heal while still building the chain gauge.