Build a smaller game
Draw a modest circle and place a manageable group of target marbles at the center. Each player chooses one shooter.
Alternate every shot
Player one shoots, then player two shoots after all marbles stop. Alternating every shot is easy to follow and gives both players the same number of turns.
Count cleared targets
Remove and count every target that fully leaves the ring. End when the ring is empty or after an equal number of shots. The larger total wins.