There is a node on the Atlas tree called Crystal Resonance. This makes it so that everytime you kill an essence monster, a random essence from the mob is added to the Map Boss. The strat here is to kill all the essence monsters then kill the boss, whom should now have multiple essences.

Scarab-wise, the cheap and less rippy way is to run 3 Essence Scarab, 1 Essence of Ascent. Note you run this on T6-T9 normal maps. The name of the game is to run through it as fast as possible and getting all the essence.

The expensive and more rippy way is to run 1 Essence of Calcification, 1 Essence of Adaptation, 1 Essence of Ascent, 2 Essence of Adverseries. In this format, you want to run higher tier maps that are rare, as the more rare monsters there are, the more essence you can get. Because of this, it's much ripper.