Very nice guide! I completed the game with your guide except I used a cold hydro build up until Captain Hunt, I also did things a little differently in the last 2 zones. You said in your guide that you're open for discussion to make a more efficient Hot Hydro build. So I want to suggest a "Warm Hydro build" for several fights in the last two zones if that's okay.
1st fight: Nostalgia
The problem with defeating Nostalgia is his Earth Titan buff. After he casts that buff, he can use an attack that scales with his focus and has a chance of 1HKO on one of your teammates. To prevent this, there is a way to stop him from casting Earth Titan completely or make him cast it when he is already at under 10,000 health. To do this you must have Flash Freeze fully upgraded, the rest of the build is completely Hot Hydro. When Nostalgia's health is just over 30,000, you can set everyone to relentless and
Flash Freeze, Wreck, Flash Freeze, Wreck, Decimate. After this he should have around 10,000 health. Now he may or may not cast Earth Titan, if he doesn't, then good for you. If he does then you'll only have to burn through 10,000 of his health rather than 30,000. If he casts Earth Titan, you can still use the Subversion trick to kill him.
2nd fight: Twin Guardians
For this fight you also need a Warm Hydro build. Except this time you'll need to max out both mind freeze and flash freeze. On the ability wheel, equip
mind freeze x2
flash freezex2
decimatex2
vapor cannon
avenger form
During the battle, make sure to have either flash freeze or mind freeze on the South Guardian, as he can use the magic bolt to really hurt you. In between using mind and flash freeze, decimate the North guardian and keep an eye out for your focus. Once the North Guardian has 3000 health, use your decimates on the south guardian but keep the flash/mind freeze combo on him. Then whittle them down to around the same health and try to kill them at around the same time. (It might help to have a flash freeze ready when you kill one of the guardians so you can freeze the other before his enraged mode kills you).
Again very awesome guide thank you for putting the time to make it inflict.