Here is a trick for getting knowledge. (Grinding for S)
If you last hit the World Boss Uturi, you can drop the knowledge..
Since this isn't so easy, just leave a Full V Tuvala Season character at Uturi's spot. Each time the world boss spawns, you farm him on season channel.
Don't try to fight with it too much, he is way too powerful for season character even if you equip bunch of HP & DR crystals.

If you get C,B,A rank, each you might want to reset the Uturi knowledge from Annolisa