Pessimistic assumption: Voldemort evaded the Mirror, and is watching every trick Harry's coming up with to use against his reflection.
Semi-pessimistic assumption: Harry is in the Mirror, which has staged this conflict (perhaps on favorable terms) because it's stuck on the problem of figuring out what Tom Riddle's ideal world is.
Pessimistic assumption: Voldemort can reliably give orders to Death Eaters within line-of-sight, and Death Eaters can cast several important spells, without any visible sign or sound.
Pessimistic assumption: Voldemort has reasonable cause to be confident that his Horcrux network will not be affected by Harry's death.
A necessary condition for a third ending might involve a solution that purposefully violates the criteria in some respect.
Pessimistic assumption: Voldemort wants Harry to reveal important information as a side effect of using his wand. To get the best ending, Harry must identify what information this would be, and prevent Voldemort from acquiring this information.
Pessimistic assumption: Voldemort wants Harry to defeat him on this occasion. To get the best ending, Harry must defeat Voldemort, and then, before leaving the graveyard, identify a benefit that Voldemort gains by losing and deny him that benefit.
Pessimistic assumption: Free Transfiguration doesn't work like a superpower from Worm: it does not grant sensory feedback about the object being Transfigured, even if it does interpret the caster's idea of the target.
Pessimistic assumption: At least in the limit of unusually thin and long objects, Transfiguration time actually scales as the product of the shortest local dimension with the square of the longest local dimension of the target, rather than the volume. Harry has not detected this because he was always Transfiguring volumes or areas, and McGonagall was mistaken.