One of the KEYS to this trial is finding the right Great Katana WS that consistently hits, does more than 400 damage, but doesn't do too MUCH dmg to your mob. Doing 5-hits then Tachi: Shoha to kill it means more time wasted finding and getting a new mob. I tried my best to fight 2-4 mobs at a time and weaponskill one I wasn't fighting. This allowed me to use 2-4 WS per mob, saving huge amounts of time. My preferred WS ended up being Tachi: Goten as it fulfilled all the requirements for finishing this trial faster.
For a seriously fast trial head to Attohwa Chasm. Here you will find Monarch Ogrefly, Cutlass Scorpion and Tracker Antlion aplenty! Repop times are 5 minutes, no wasted stones using Abyssea, and you can get geodes to drop during day/weather, which makes it really nice.
The Boyahda Tree is also a good choice. Grab page 6 from the Grounds Tome, head to the 3rd map, and kill 3 Darters - the left-hand corner once you drop down from the waterfall is free from Goobbue, Leech, and Mandy aggro. Put up sneak, drop down to the southern section, swing around to the tunnels leading back up, and kill 8 Processionaires on your way back to the second map. Put up Sneak again, run down the waterfall, kill 6 Darters, and repeat. Can usually get about 3 WS per mob. There's a Grounds Tome right in your path, too, if you want to keep Regen or Reraise up. All you have to worry about is Elementals, but if you avoid bringing a magic-using subjob, you're fine. Item drop rate on the mobs is pretty low, too, so you won't have to worry about clogging your inventory.