Alchemy Essence is the main Evo material of the Fullmetal-themed update in Anime Last Stand. Every new unit added in Update 66 requires this resource for its evolution. The good news is that the drop is guaranteed on the right stage. So, to help you get and use Alchemy Essence in Anime Last Stand, here's a complete guide on everything you need to know.
How to Get Alchemy Essence in Anime Last Stand

Queue the Laboratory 5 - Legend stage. Essence has a 100% drop rate here, awarding 20-35 Alchemy Essence per clear. Difficulty is quite high for this stage, so you will need endgame teams to clear this stage.
You can run this stage only on solo. All the new evos in this update typically require around 200-250 Alchemy Essence each for their evolutions.
How to Use Alchemy Essence in Anime Last Stand

Alchemy Essence is spent directly in evolution recipes for Update 66’s units. Since all newly added units require Essence, this is the material you will run out of first. If you intend to evolve one copy of all six newly added units, expect a total cost of ~1,300 Alchemy Essence across their Evo recipes.
With Laboratory 5 awarding 20-35 Essence per clear, that works out to about 65 clears in a worst-case run and roughly 48 clears on an average reward. So, you should plan your grinding session accordingly to get the most value.
Frequently Asked Questions (FAQs)
Only from Laboratory 5 - Legend Stage. Each full clear grants a guaranteed Essence reward between 20 and 35.
Update 66 Evos like for the Short Alchemist or Armored Alchemist range from 200 to 250 Essence per unit. If you want to evolve one copy of all six new units, you'll need 1,300 Essence total.
For more on Anime Last Stand Roblox, check out How to Get and Use Homunculus in Anime Last Stand or How to Get and Evo Armored Alchemist in Anime Last Stand on Pro Game Guides!