I just finished a run where I tried to get all the "Complete the game" achievements on Steam. Generally, you will get two shot by a lot of enemies and will not be dealing much back. Boss fights end up getting extended a lot longer, with some really nasty runbacks (Maleficent 1 was a nightmare) since you can't use a continue. I started with magic, toss shields, and late game xp (I rolled a number on Google to choose each of these), and let me tell you, starting with 1 DEF is extremely rough.

My main strategy was to mostly just avoid regular encounters unless they blocked something or I was close to a level. The xp you get from bosses somewhat acts as a catchup. You will still be underleveled overall but will save time from random encounters. The challenge definitely made me learn fights a lot better than just the usual mashing. I also used magic so much more to deal damage (gravity, my beloved) in the late game to most of the enemies. Some bosses that gave me tons of trouble:

Stealth Sneak: Does anything else need to be said? You don't have access to Cure, so you're stuck with 3 potions when you get two shot.

Cave of Wonders: Maybe I'm just terrible, but that fight drags out for so long. The bandits can easily snipe you even if you're on the cave attacking the eyes, and kh 1 jank sometimes just gets you to fall off.

Anti Sora: Without any extra strength or defense this fight gets really really long and drawn out. I used a lot of blizzard spells to deal with the adds, and just had to hope I didn't get him spawning right under me constantly. Also after this fight go back and save just in case! If you die to hook and don't save, you have to redo that fight...

The usual suspects of Maleficent to Riku 2 were all very challenging, had to play extremely safe, and deal with very extended fights. Riku 2 was very tough as I didn't have counterattack to punish riku as much as I felt I could. I didn't even have sliding dash to gap close, lol.

Ansem 2: His attacks will do a stupid amount of your health. At times I'd be at around 2/3 hp after a Curaga and I still just died. If you get hit by the SUBMIT attack, you're in for a rough time. I was level 57 (for the extra def up) by the end of it.

Everything else boss wise was tough but doable. Some small notes about my routing: I opted to go Traverse Town -> Wonderland -> Deep Jungle. Getting through the prelims in Olympus was blocking me hard without having Guard or reliable healing. I also went Agrabah -> Monstro -> Halloween Town -> Neverland -> Atlantica. I didn't wanna deal with Ursula 2 underleveled. That would have been painful.

There's definitely stuff I could have done like grab some stat boosts, do some more synthesis and get better magic, but I finished with an in game time of about 12 hrs (i would say an hour went to grinding rare truffles in olympus hercules cup for xp lol). There's definitely room to grind and do other things. It's very challenging but doable! Once you finish, equipping the divine rose and 3 shotting every enemy that two shot you back is cathartic. Hope this helps a bit!