When Necromancy enters the battlefield, it triggers, returning Astral Dragon from your graveyard to the battlefield, and then attaching Necromancy to Astral Dragon.
When Astral Dragon enters the battlefield, it triggers, creating two token copies of Necromancy that are 3/3 Dragon creatures in addition to their other types.
When the Necromancy tokens enter the battlefield, they trigger, with at least one trigger targeting Glasspool Mimic.
Resolve the first Necromancy trigger, returning Glasspool Mimic from your graveyard to the battlefield as a copy of Astral Dragon, and not attaching Necromancy to Glasspool Mimic due to Necromancy being a creature.
The Necromancy Aura creature token is put into your graveyard as a state-based action due to being an Aura that is unattached.
The Necromancy token leaves-the-battlefield ability triggers, as well as Glasspool Mimic's enters-the-battlefield ability.
Resolve the Necromancy trigger, causing you to sacrifce Glasspool Mimic.
Resolve the Glasspool Mimic trigger, creating two token copies of Necromancy that are 3/3 Dragon creatures in addition to their other types.