Oh boy...

G2 Akkan, can be done Solo.

Only one of the two counter patterns counts, and it's totally random. I spent 90 minutes to farm 64 counters (he did 74 of the wrong ones).

Equip defensive engravings (I used Fortitude, Heavy Armor, Divine Protection, Strong Will, Emergency Rescue), take off Pants and Paradise Orb. Select the extra Sidereal Gauge bonus and spam Innana to lower your gauge + heal. Don't reset on enrage, you can survive it (1680+), it actually helps to speed up shit patterns.

He counters around every 45-60 seconds on average. Half of them are the wrong counters. I've had it take up to 4 minutes between counters even in berserk. He can also counter back-to-back within a few seconds. Expect it to take over 2 hours to farm this achievement on average.