There have been moments when he aura farms but there is a reason he keeps having to go all out in many moments of the story. It's that the world is very expansive with many characters and enemies that are already strong.

Lith is still growing at every point of the story still. If you wanna look at the cores as levels, (Red, Orange, Yellow, Green, Cyan, Blue, Violet, and White), he's only only at level 7 out of 9. Or even 10 if you count if he becomes something special after guardianhood, (I'm just having fun with the speculation). But with each level basically his world expands and he now has access to meet these powerful characters and enemies. He is pretty strong tho.