To Complete Do Your Worst, each player must have a unit in the chosen arena.
When the untap step ends, choose an arena. Each player chooses one of his or her units in the chosen arena. Then roll a die. If you roll a 1, tap your chosen unit. If you roll a 2, tap both units. If you roll a 3, 4 or 5, tap your opponent's chosen unit. If you roll a 6, discard your opponent's chosen unit.