Astral Dragon + Necromancy + Clever Impersonator

Steps

1. Cast Necromancy by paying {2}{B}.
2. When Necromancy enters the battlefield, it triggers, returning Astral Dragon from your graveyard to the battlefield, and then attaching Necromancy to Astral Dragon.
3. 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.
4. When the Necromancy tokens enter the battlefield, they trigger, with at least one trigger targeting Clever Impersonator.
5. Resolve the first Necromancy trigger, returning Clever Impersonator from your graveyard to the battlefield as a copy of Astral Dragon, and not attaching Necromancy to Clever Impersonator due to Necromancy being a creature.
6. The Necromancy Aura creature token is put into your graveyard as a state-based action due to being an Aura that is unattached.
7. The Necromancy token leaves-the-battlefield ability triggers, as well as Clever Impersonator's enters-the-battlefield ability.
8. Resolve the Necromancy trigger, causing you to sacrifce Clever Impersonator.
9. Resolve the Clever Impersonator trigger, creating two token copies of Necromancy that are 3/3 Dragon creatures in addition to their other types.
10. Repeat from step 4.