Mardu Siegebreaker, and Felidar Guardian

Prerequisites

• Mardu Siegebreaker on the battlefield
• Felidar Guardian in exile

Steps

1. Declare Mardu Siegebreaker as an attacker, triggering its second ability, creating at least one token copy of Felidar Guardian that is tapped and attacking
2. When the Felidar Guardian token(s) enter the battlefield, they trigger
3. Resolve the first trigger, blinking Mardu Siegebreaker
4. Mardu Siegebreaker leaves and enters the battlefield, causing the nontoken Felidar Guardian to return to the battlefield from exile, and triggering Felidar Guardian and Mardu Siegebreaker's first ability
5. Resolve the Mardu Siegebreaker trigger, exiling the nontoken Felidar Guardian until Mardu Siegebreaker leaves the battlefield
6. Resolve the Felidar Guardian trigger, blinking Mardu Siegebreaker
7. Repeat from step 4

Results

• Infinite LTB
• Infinite ETB