If you have finished the game you will learn that there is no way back to any past locations to farm Lacrima and level up.

A message after you defeat the second boss in Demo will tell you that whatever Skill Tree and Stats Upgrades you got will be transferred to the full version in the release day.

But you can't go back to any past locations and farm Lacrima so you have to start a new game and farm as much as you can and then finish the demo again.

Before First Boss

The save point before the first boss fight is the best place for Lacrima.

Right before the small path to the first boss fight you can go right and fight a mini-monster which will give you 480 Lacrima.

This takes around 15 seconds if you only kill the mini-monster.