Low 50s still a dead zone for large monipulators?

Low 50s: beetles and scorpions in Western Altepa.

Lower Middle 50s: Antican PLDs and BLMs and the one roc around D-11 in Western Altepa are working very well for 53 and 54. Antican BLMs don't cast Ice Spikes. Manticores may be useful too if you're not a lizard.

54~59: Esbats and Glacier Eaters are ridiculously weak for their levels. They're in Ulegeurand, in the tunnel with the orc NPC.

58+: Caedarva Mire, saplings and birds near the entrance to Nashmau. Orderly imps are nice too. Watch out for chigoes if you're not a lizard.

I edited the post. I've figured out a nice routine for 50-60.

Its not hard to level large mobs if you put some effort into it! O.o