Coming back to this fight, I had to de-rust a bit since I haven't touched KH3 for a couple days from the initial attempts. After a few practice runs, I got more consistent getting to the last 2 HP bars and with the altered phase 2 patterns (still not perfect though). However, I still can't for the life of me get used to how quick that keyblade strike pattern is and the Desperation Move.
During a failed attempt earlier, I accidentally found an opening during the Desperation Move. There's an opening at the very end of the second round of attacks and I accidentally found it out by having Double Form on and proccing the evade-attack ability which ended up hitting Terranort just before he slammed down the last hit of his DM. However, I wasn't able to recreate that in this attempt because of the stress and timing required from me to pull it off without mistiming it.