Magar of the Magic Strings + Last Chance + Obeka, Brute Chronologist

Prerequisites

An opponent cannot block creatures you control.

Steps

1. Cast Last Chance by paying its mana cost, causing you to take an extra turn after this one and to lose the game at the beginning of that turn's end step.
2. Activate Magar by paying {1}{B}{R}, noting Last Chance and putting it face down on the battlefield as a 3/3 creature.
3. During your next turn, deal combat damage to an opponent using the face down creature that has Last Chance noted.
4. The creature triggers, casting a copy of Last Chance without paying its mana cost. Resolve the copy, causing you to take an extra turn after this one and to lose the game at the beginning of that turn's end step.
5. Activate Obeka by tapping it, causing you to end the turn. This will prevent the Last Chance trigger from causing you to lose the game.
6. Repeat from step 3.