Boros Reckoner, and Fiendlash

Prerequisites

• Boros Reckoner on the battlefield
• Fiendlash on the battlefield
• Fiendlash attached to Boros Reckoner
• You have a way to give Boros Reckoner indestructible
• You have a way to deal damage to Boros Reckoner

Steps

1. Give Boros Reckoner indestructible by any method
2. Deal damage to Boros Reckoner
3. Fiendlash and Boros Reckoner trigger
4. Resolve the Fiendlash trigger, dealing at least five damage to any player or planeswalker
5. Resolve the Boros Reckoner trigger, dealing at least 1 damage to itself
6. Repeat from step 3

Results

• Infinite damage