Activate the creature with Elemental Mastery attached by tapping it, creating a number of 1/1 Elemental creature tokens with haste equal to its power.
Activate Shilgengar by sacrificing an Elemental.
When the Elemental dies as an artifact due to Encroaching Mycosynth, Dross Scorpion triggers, untapping the creature with Elemental Mastery attached.
Resolve the Shilgengar ability.
Repeat.
Encroaching Mycosynth turns everything on your board into an artifact, which is the secret glue that lets this engine hum. Because of that passive effect, whenever you tap the creature enchanted by Elemental Mastery to flood the board with hastey Elemental tokens, sacrificing one of those tokens to Shilgengar, Sire of Famine counts as an artifact death. That triggers Dross Scorpion to immediately untap your token generator, letting you loop the process infinitely while piling up Blood tokens and churning through your battlefield.