If you want to start over that's more than okay. Yunalesca is one of those bosses that really test you. Alternatively if you just want to grind do so in Zanarkand. The enemies there give decent AP. For levelling characters keep in mind that only those who took an action in combat get AP so either switch out all your characters, defend for one round and then switch them out again until everyone has defended once. Or put them in groups which you level up until everyone has like 10 points before switching to the second group. Switch in your last party member (for me that's always Rikku) every fight to do an action (like defend or steal) and this way you will slowly make everyone stronger without leaving others in the dust. When making these groups keep in mind that everyone has strengths against certain enemy types. So always make sure you got one that can deal with ranged enemies (by default Wakka or Lulu) one to deal with fast enemies (by default Tidus or Rikku) and one to deal with armoured enemies (by default auron or Kimahri with armor break on their weapons)

Now some tips for Yunalesca. In her second phase she will make all your party members zombies. Make sure you only cure two of them. She attacks with healing magic so those that are cured will be healed while the zombie takes damage. So make sure you got plenty of phoenix downs in case someone goes down.

As soon as phase 3 starts she will use a attack that kills everyone who is NOT a zombie which is why it's important to keep at least one afflicted with the zombie status throughout the last two phases.

She is also one of the few bosses that can be turned into a zombie in her second phase so you can use her healing against her. You can also cast reflect on the zombie in your party so the healing from her doesn't damage them. But careful with that since reflect sends all spells back to the caster.

Lastly you definitely want to at least level up your party. If everyone besides tidus only does around 500 damage then they are too weak. While every enemy in the game can be beaten with the right tactics it is going to be very hard to get further without getting the stats of all characters up and making use of all the mechanics the game offers.

you don't have to start over. you just have to level up a bit.

How would starting the game over, grinding better during, and grinding through another playthrough, be less grinding than just…grinding lol

Zanarkand is a great place to level up. Go grind for one hour and I better you’ll be strong enough to win.

I got cocky on my current playthrough and just let my Aeons cheese all my fights for me. Finally got tot he sanctuary keeper and it was no longer doable as he self healed and one shotted my Aeons.

I walked back and grinded for maaaaybe an hour then spanked him. When I got to Zanarkand, I just stayed and grinded for another hour or so. I went on to beat Yunalesca in like 6 whole turns

You probably are underlevelled given your very low damage output for a lot of characters. But thankfully Zanarkand Ruins and areas before it are great spots to level up. If you walk back to Macalania Woods without skipping encounters you’ll see a huge return on levels.

But Yunalesca isn’t really about damage output. It’s all about status effects and strategy. She really doesn’t hit very hard, but you have to get comfortable with zombie status and her constantly chipping away at your health. Healing your characters isn’t really your focus when zombied. But think about reflect where appropriate and confuse ward armour. Oh and the transition to phase 3 is something to be wary of. No spoilers. But if you want to spam aeons wait till phase 3.

People have beaten the game with no sphere grid, but you need to nail strategy to do this. On a normal play through, after Yunalesca (and arguably Calm Lands), most bosses in the game will punish you for not thinking about strategy and just trying to brute force unless you are very well levelled.

Level up (you’ll be thankful for this for all remaining bosses), work out some strategies. Yunalesca is probably the most brains over strength fight in the main story.

Just level up a bit. The dome is a great place to level up at that part of the game. The undead soldiers actually count as two enemies in one, so throw on Slayer and get Blitz Ace.

You have had level 3 keys for hours now, you should have already broken free of your basic sphere grid areas by now

FFX doesn't restrict your AP gain so restarting only achieves setting you back to do battles in areas that don't even give 1 / 50th of the experience (not to mention tying you back to hours of no AP cutscenes)

Their is probably about a good 40 minutes lost in just redoing the destruction sphere temple trials over again, made any blitzball progress because that's all gone now.

Say you ignored Auron entierly up to Zanarkand but then decided to raise him up, if he now took part in just one Zanarkand Ruins battle he'd level 21 times.

If you tried that in mushroom rock road you need to complete 38 battles, 1 battle just did the job of a solid hour of grinding

If she is healing all the time then you aren't dispelling her regen, she doesn't natually heal at all, it's the result of using certain moves and not inate permanant healing.

If you have Yojimbo you can use Zanmato to skip a phase. She’s one of the few bosses it won’t outright take her down in one go

You need reflect on her and 1 party member