I was messing with side quests using all high damage spells, making it more likely to kill quickly but also more likely to die, and was noticing that abnormally high amount of exp it was giving when 2 enemies would die and the 3rd would kill. This is because side quests and main storyline has an increased amount of exp compared to normal battles, which is fine.
The problem is this buffed exp amount is still given even when only 2 of the 3 units are killed. This makes the most efficient way to farm exp to be to find a side quest that can 2 shot the player where the player can 4 shot or less 2 enemies, then cast 2-4 spells to kill the units, then die to the 3rd.
This can be done as low as level 1, all the way until level 50 and citadel, and gives around 2x-3x more exp than any other method I am aware of.
This is very similar to the dungeon issue STILL in Gems of War for over 2 years now, in that farming a level 20 dungeon at 1x multiplier by never raising the dungeon level causes the highest dungeon loot in the game. The big issue with this in PQ3, similar to Gems of War, is that once every questline and side quest is completed on an account, this method is no longer doable, similar to how when a person raises all their dungeon above level 20 in GoW the method also stops.