i did that trial in den of rancor >: at the zone to that hakutaku spawn area.. 5 slimes at each of the ways with 16min respawn
boring and kinda long but really easy
i liked to just afk and do rl stuff but if you want to keep going theres a lot more slimes close mixed with other mobs
There's a spot in Garlaige Citadel (S) with 8 rooms, four on each side of a hallway, each containing a slime. They die easily with a Blizzard5 --> Thunder4. The respawn is awhile, but you can watch a movie or do homework at the same time.