Elemental Mastery, and Midnight Guard

Prerequisites

• Elemental Mastery on the battlefield
• Midnight Guard on the battlefield
• Elemental Mastery attached to Midnight Guard
• Midnight Guard does not have summoning sickness

Steps

1. Activate Midnight Guard by tapping it, creating two 1/1 Elemental creature tokens with haste
2. When the Elementals enter the battlefield, Midnight Guard triggers twice, untapping itself
3. Repeat

Results

• Infinite creature tokens with haste
• Infinite ETB