This oversized robot is one of the strongest Guardians of the Tower of Harvestasha, and it will attack the party while Akane sings EXEC_METEMPSYCHOSIS/. to open up the Rinkernator and revive Tyria. That's why the battle against this monster is called the Battle of Destiny: you have around five minutes to destroy MYU. If you manage to do so, you will be allowed to choose between the Normal and True Endings at the end of Phase 3, but if you run out of time, Saki and Finnel won't be able to continue singing due to the lack of Symphonic Power, which will ruin any possibility of reviving Tyria and thus, you will be able to choose only between the Normal and Bad Endings.
Therefore, if you plan to proceed to Phase 4 and attain the True Endings, make sure to defeat it, although it won't be an easy task: MYU has an enormous amount of HP regardless of difficulty and it's nigh immune to Song Magic attacks, so even a well-charged Flipsphere won't be enough to kill it. Likewise, it has a very strong defense and attack power, and its physical attacks, which consist of stomping the ground to generate shockwaves and launching the missiles on its wings, hit multiple times for high damage and can cause all status ailments on all characters they hit. And as if this wasn't enough, its Stored Attack: Ar tonelico Ver. 1, in which it joins its wings to turn them into a laser cannon and fire a beam which releases explosions upon impact, causes large damage to all party members, so if you don't keep your HP high, it will undoubtedly kill off all of your characters. And interrupting this attack is pretty much impossible in higher difficulties, unless you have your inventory stocked with attack items.
The best way you will have for defeating this mechanical monstrosity is using the Ultra Supermoves, as MYU's physical resistance is pretty much nonexistent during the initial stages of the battle: Purge your Reyvateil to Lv. 4, let the Burst accumulate for a while, and then unleash all of the skills of this type you have synthesized at this point, which if you have been diligent about synthesizing items, you should have them all. Keep on mind that once MYU's HP has dropped to 45% or less, it will activate an special passive skill called Mode Shift, in which it becomes extremely resistant to physical attacks, all elements and also gains extra attack power and defense. However, the Mode Shift has a fatal flaw: it reduces its Song Magic resistance to almost nothing, so once he has changed modes, feel free to unleash a Flipsphere to finish it off. All in all, you should be able to finish it off with plenty of time to spare.
As a final note, you can keep track of how much time you still have left not just by looking at how many of the Rinkernator's walls have opened, but also by looking at the colored lines that are located at the borders of the battlefield, one being right below where your Reyvateil is singing. These lines change from blue to yellow, and then to red to indicate how much time you have left. However, if the line turns red, that means you have less than a minute to finish up the battle, so unless you want to get the Bad Ending, make sure to hurry up and finish MYU off at once.