Alright, good news-bad news.
Bad news is that if you think you can just play the game on Normal, do absolutely no side content and then just change difficulty before the final boss then you have another thing coming.
While you can play the entire game on Normal and just change to Hard before the final boss,
is a very hard and long battle.
If you can't cut it in other parts of the game on Hard, you absolutely won't be able to do it here either.
If you don't want spoilers on the identity of the final boss don't read the following.Good news is, this fight is totally doable; it just takes a little preparation.
Even if you can't use the Costly Punch tactic, I've got you covered.
Genesis is a two part fight but the tough one is the first, Genesis Avatar.
Bring along a couple of Phoenix Downs to be sure but here's exactly how I did it with a lvl 43 Zack and about 50% of all missions done.
Genesis Avatar uses a couple of moves but there's only one we really need to look out for and that's Shadow Flare which can kill you immediately even at full health.
He does this every time he recovers his sword so always guard until this attack is over.
For the rest of the fight, it's pretty straightforward.
You can't damage Genesis Avatar himself because he's out of range.
You can damage him with Flare but just keep the focus on damaging his sword whenever he brings it down.
He will also summon a horde of enemies each time. Now, you might be thinking that this is bad but this is what allows us to beat this battle.
Due to our magic absorption, every time these guys attack us with magic, we're just going to let them as this will heal us.
Also, if you see the Shadow Knights using Black Thunder, do your utmost to get hit by it.
DO NOT avoid it; this will refill your MP because of the magic absorption.
Out of the three mobs he spawns, which are Shadow Scythes, Shadow Knights and Shadow Mages, the only ones that are useless to us and should be dealt with immediately are the Scythes.
The rest will either heal our HP or MP.
Keep in mind though that they still pose a degree of danger.
Avatar will also slam down his sword for some small area damage which can be easily dodged and later in the fight he can use Overdrive, which can rob you of MP and AP.
This is where Osmoga comes in especially useful. Don't bother with draining the Shadow mobs or Avatar himself (unless you have completely drained the sword) as the Materia sword has a huge pool of MP.
It's not infinite but it can get you by until he's at about 25% of his health.
My equipment was:
Electrocute (or Hell Thundaga or anything that suits you for offense)
Quake (which I never used once so subtitute it for something you like. I recommend Lucky Stars or Octaslash)
Flare (again, not necessary if you use Electrocute. Pick whatever suits you.)
Curaga (must)
HP UP++ (if you are not at 9999/9999 don't even consider going. You should always be full health in this fight.)
Osmoga (must)
Shinra Beta+ (or your preference)
Wizard Bracelet (Absolute must. Sector 7 Shop or Network Shop Duo for a hefty 80.000 gil. Sector 7 is found in M4-4-3 as a treasure chest while Duo is a treasure chest in M9-3-3 but you probably won't have this.)
Elemental Blade (you can use something better here like anything that improves Luck.)
Moon Bracer (very important)
Remember to always use MP Cost zero to spam Electrocute on the sword. Keep your health up at all times. Prioritize MP for Curaga over Electrocute.
Avatar will also use an unavoidable attack called Purgatorial Wave. If you are at full health it shouldn't be too bad; just heal immediately after.
At some point he will stop casting Shadow Flare which is by far the worst attack he has and he will replace it with regular flare. Just guard and you'll be golden.
It really isn't too hard a fight, just extremely long.
His pattern is pretty much the same too.
Brings down sword for you to attack, summons mobs, takes sword up, uses Flare or Shadow Flare, a Purgatorial Wave to mix it up and that's it.
Now, for part two you get Genesis himself at his 100%. He's much quicker to defeat but he's more dangerous in the sense that he can kill you quicker too.
But really, he's not too bad and even if you do fall, you can just retry the battle without going back to Avatar. (Hooray for QoL improvements!)
If you do die, just take out the Wizard Bracelet as it won't help you at all in this fight.
Spam him with Electrocute and he's toast.
Congratulations on beating the game on Hard!
Now go watch one of the best endings in gaming history.