I recently came back to the game after being gone and having not played any expansion past serpents spine really. I took a group of Autogrant AA, Augless, Attunable only garbage characters to 120 without much of a plan at all. Hope this helps.
80-90 in Rathe Council Chambers
90-95 Morell's Castle (Many posts about this on RG)
95-100 in Beasts' Domain (Farming in the field, random mobs)
100-105 in Plane of Shadow (Yolo went to a zone, set up a camp and didn't move)
105-110 in temple of droga (Again, picked a zone randomly and set up a camp. this was by far the fastest 5 levels)
110-115 in Sathir's Tomb
115-120 using mq2grind in GD mission.
This was done 100% afk utilizing nothing but the automation available on redguides. I only played the characters when I was finding a new spot to grind in, clearing to a new camp etc. I upgraded gear every 5 levels using the remnant gear in plane of health. After 105, the group got one round of upgrades for attunable 110 gear then nothing else after that. The group was all gold membership so they got all the AA's every time they leveled but no one had any AA's after 110.
Terrible group comp too, I only leveled them for buffs. (War/Clr/Shm/Mag/Enc/Rng)