Mercurial Pretender, and Felidar Guardian

Prerequisites

• Mercurial Pretender in hand
• Felidar Guardian on the battlefield

Steps

1. Cast Mercurial Pretender by paying its mana cost, choosing to have it enter the battlefield as a copy of Felidar Guardian When Mercurial Pretender enters the battlefield, it triggers, blinking Felidar Guardian
2. When Felidar Guardian enters the battlefield, it triggers, blinking Mercurial Pretender
3. Choose to have Mercurial Pretender enter the battlefield as a copy of Felidar Guardian
4. Repeat from step 2

Results

• Infinite LTB
• Infinite ETB