I thought the Tread Leeches (Lv.19-22) would be relatively easy to charm? Carnivorous Crawler (Lv.20-23) wouldn't be too bad, either.
Those can hold their own against other leeches and crawlers for half a battle, I'd think, significantly reduce down time. As bad as losing TH would be, sitting there to regen HP all the time sounds terribly boring.
To OP:
You can always check
FFXI Atlas or
FFXIClopedia to see what's the level gap between you and the monsters you want to keill.