Grinding locations based on level.

Lv 24: Malevolent Skeleton Archer (Gludin Highway) - Drops Moretti Cores & Quest Scrolls. Grind this place overnight when you're stuck.
Lv 25: Arachne (Elven Ruins Catacomb 1) - Mob train. Drops PoBs and Arachne Core.
Lv 35: Shaper (Elven Ruins Catacomb 2) - Mob train. Drops PoBs and Necross Core.
Lv 36: Ant Soldier Larva (Ant Nest Catacomb 1) - Mob train. Drops Reaver Core.
Lv 46: Ant Soldier (Ant Nest Catacomb 1) Party spot.
Lv 50: Ant Captain (Ant Nest Catacomb 1) - Party spot. Drops PoBs and Ant Nest Foreman Core.
Lv 54: Ant Larva (Ant Nest Catacomb 2) - Mob train.

Fields of Dion is when you start to actually feel the effect of your hard work. Enchantment Scrolls, Maphr's Protection, and A gear starts dropping more frequently. Monster Cores also become a byproduct of your field grinding.

Lv 62: Baroque & Breka Orc Ranger - Drops Baroque Core & Quest Scrolls.
Lv 66: Screash & Leto Lizardman Shaman - Drops Screash Core & Quest Scrolls.
Lv 70: Breka Orc Infiltrator - Drops Bulc Core & Quest Scrolls.

And now we're back to Elite Dungeons.

Lv 78: Ant Soldier Royal Guard (Ant Nest Catacomb 2) - Mob train & party spot. Drops PoBs and Ant Nest Royal Guard Captain Core.
Lv 84: Mordeo (Cruma Tower Floor 2) - First spot after the entrance. Lots of people AFK here.
Lv 94: Master Mordeo & Mordeo's Guard (Cruma Tower Floor 2) - Mob train. There's always at least one party here and a few randoms at any given time. If you can hit hard by yourself and you can take a few hits, you can probably get away as a level 70 here. Drops Master Mordeo Core.
Lv 100: Mordeo's Mirror Image (Cruma Tower Floor 2) - Drops PoBs.

For Cruma Swamp, there is only worthwhile codex to grind here due to the abundance of its spawn.

Lv 84: Swamp Killer - Drops Morgan Core & Quest Scrolls.

Next up is Summit of Dissonance. You'll be staying here for a while because there are a lot of good Codex monsters here.

Lv 90: Black Leopard - Drops Summit Attacker & Quest Scrolls.
Lv 90: Tanor Sentinel - Drops Tartan Core & Quest Scrolls.
Lv 94: Kankoon's Elite Soldier - Drops Thaartan Core & Quest Scrolls.

The last map for this patch is Shrieking Hallows so try to complete all the Codex monsters here.

Lv 104 Tortured Mandragora - Drops Screamer Core & Quest Scrolls.
Lv 104 Dicor of Sorrow - Drops Flora Core & Quest Scrolls.
Lv 112 Resurrected Specter - Drops Cowboy Core & Quest Scrolls.
Lv 112 Doom Servant - Drops Jonadan Core & Quest Scrolls.
Lv 112 Ominous Willow - Drops Shrieking Treant Core & Quest Scrolls.
Lv 116 Nerkas Necromancer - Drops Balthazar Core & Quest Scrolls.
Lv 116 Relentless Specter - Drops Seamstress Core & Quest Scrolls.

Cruma Tower Floor 3

Lv 112 Validus Guard - Drops Grezo Core.
Lv 120 Deranged Meforde - Drops PoBs.
Lv 128 Rorka - Drops Magnus Core & PoBs