There are two points in which Harvestasha is fought, although you will be able to get only to one of them depending on your actions: one of these is Harvestasha being the final boss in the game, which only happens in the Normal Ending Routes; and the other is fighting her in Phase 4, which only happens in the True Ending Routes.
In this path, Harvestasha has a high resistance to all elements excepting Thunder, is immune to all status ailments and takes only half damage from Song Magic. However, she has a pitiful physical resistance and the Guardians that accompany her are S-01s, which while strong, they shouldn't be much problem to defeat with Song Magic. However, watch out for Harvestasha's Stored Attacks: one is Cacophony, which stops the growth of the Burst Gauge for 20 seconds, which can get you in severe trouble if you need to release the Song as soon as possible. However, her second attack is even more dangerous, and you should recognize it if you have played the previous games: it's Ar tonelico, in which she launches large torrents of the Symphonic Power stored in the Tower of Harvestasha to deal large damage to all your party members. Considering this, you should do your best to interrupt her whenever she tries using any of them.
For defeating her, the best you can do would be first charging a Song to a 20000% Burst and getting your Reyvateil to Lv. 4 Purge, which should allow you to wipe off all of the S-01s. After that, surround her with all your Vanguards and continue attacking her until you can use a well-charged Flipsphere or Ultra Supermoves to finish her off. Keep on mind that all of Harvestasha's normal attacks only hit the characters directly in front of her, so try to attack her from the sides or behind to not get hit.
On the opposite side, while this version of Harvestasha has the same Thunder and Song resistance as the Normal Route one, she also got a reduced reduction to the other elements but an increased resistance to physical attacks. Additionally, the monsters accompanying her are now K-21Ps, much stronger than the S-01s, and to make matters worse, if you manage to kill them, Harvestasha will summon S-01s as replacements, so this time around, you won't be able to keep her surrounded.
Therefore, the best you can do is making sure to interrupt her whenever she attempts to use her Stored Attacks, and keep yourself alive and attacking until your Reyvateil is at Lv. 4 Purge and you have enough Burst accumulated to launch the Ultra Supermoves and the Flipsphere.