NINJA GAIDEN Σ

You can get to 99999 anywhere, but an easy farming spot is the Chapter 12 Aquaduct of the pillbugs ambush 2nd time around and doing UTs and not killing all pillbugs then going the elevator and then back down the elevator repeat

You are unlikely to earn this achievement unless you specifically try for it, or are not using any Essence at all for the majority of the game and killing lots with Ultimate Techniques. The best method that I have found is to head to Drawbridge Square in Chapter 16, where there are the 3 Military Ghuls. Each of these if killed with a UT is worth about 7000 Essence, without any boosts. They respawn if you leave and come back to the area, so after they are killed, use the door with one blue dot on and then go back, they will have respawned. Even from 0 Essence, 5 times doing this will get you 99999.

Equip the holy gauntlet (unlocked mission 4) and the armlet of benediction. On chapter 18, farm the ghost fish left of the first save point. Takes maybe 5-10 minutes to cap out. Just use flails and absorb all the essence - blue, yellow and especially Red. Takes no time at all.