Ok first off there are certain spots where mobs respawn fast and there is plenty of them other than coblyns/doblyns in the mines, and yes I form my grind parties. All you have to do is go on yg and look for mobs 10-15 ranks above your party.
Rank 12ish group - Northeast of camp bloodshore (jellyfish, flytraps, moles)
Rank 18-23ish - Coerthas- Southeast of camp glory (deer, wolf, humans), Humblearth - south of gridania (deer, mouse, crabs, bats, elementals)
Rank 24-27ish - Move up near Camp Dragonhead (deer, yarzon, puks, goblins hippocerf)
Rank 26-29ish - Camp Bald Knoll (Crabs, birds, mouse)
Rank 28-35ish - Mistbeard Cove (Efts, pudding, pirates, ahrimans, yarzons)
Rank 33-37ish - Bloodshore - east of camp bloodshore (Goats, Rats, flytraps), I suggest you leave the apkallu alone.
Rank 37-42 ish - Coerthas - north of zone to mor dhona (antelope stags)
Rank 44ish - raptors and deer near boulder down if you're brave enough. Surf Eft and birds in Zelma's Run.

So yeah, you don't have to be restricted to doing broken water leves to 50. That's not even a complete list since I hate exploring gridania area.