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.
Activate Magar by paying {1}{B}{R}, noting Last Chance and putting it face down on the battlefield as a 3/3 creature.
Activate Magar by paying {1}{B}{R}, noting Time Stop and putting it face down on the battlefield as a 3/3 creature.
During your next turn, deal combat damage to an opponent using the face down creatures that have Last Chance and Time Stop noted. The creatures trigger.
Resolve the first trigger, 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.
Resolve the second trigger, casting a copy of Time Stop without paying its mana cost. Resolve the copy, causing you to end the turn. This will prevent the Last Chance trigger from causing you to lose the game.
Repeat from step 3.

Infinite turns

Magar of the Magic Strings

Time Stop

Last Chance

During your opponent's upkeep step, Seedborn Muse untaps all permanents you control.
Cast Time Stop.
With Time Stop on the stack, activate Isochron Scepeter, casting a copy of Narset's Reversal.
Copy Time Stop with Narset's Reversal, returning Time Stop to your hand and ending the turn.
Repeat on each opponent's turn.

Combos with "Time Stop"