At the time Arcana summoned the 'Malice Doll of Demise', Yugi and Arcana had 1500 life points. Yugi had a Big Shield Gardna on the field at the time with 100 Attack and 2600 defense. When Big Shield Gardna is attacked, he changes to attack mode. If Arcana attacked with Dark Magician which has 2500 attack, he would lose 100 life points but would force Big Shield Gardna into attack mode. Arcana could then attack with Malice Doll of Demise which had 1600 attack and win. But instead, elected not to for no particular plot reasoning as Arcana desired to win quickly.