Though getting to lvl 85+ does help, it is possible to beat sephiroth at alot lower level. The first time I beat him I was 54. Here are some tips I used on countering most of his more devastating attacks.
Abilities required:
guard
counterguard
retaliating slash
high jump 1/2/3
glide(i did it without, but it helps)
aerial recovery
trinity limit
Items:
As many elixirs as you can carry, in my case it was 6, but I only used 3.
Problem:
"Descend, heartless angel."
Also referred to as "Sin Harvest", this attack is almost always followed by the reaction command attack of Sephiroth's, which leads to instant death.
Answer:
Trinity limit. Important you execute this BEFORE you lose all your health and MP. If you don't have the MP for it then use an elixir and spam the triangle button to block Sephiroth's finisher. Note: this is not the only attack where sephiroth soars high into the air. Do NOT mistake the meteor shower for this attack or you will waste your MP. When he calls down the meteors you can't touch him, and if you hit the trinity you'll only get an embarassing combo attacking the air. Also be sure to LOCK ON to sephiroth before you use the trinity, or else you run the risk of trinity limitting the air in front of you, thus wasting your MP.
Problem:
Ground melee attack combo of death.
Answer:
This one is simple. Use guard and counterguard. Guard when he comes in, then hit the counterguard at the end of your guard. This will break sephiroth out of his combo and put you into one of your own. This requires you to judge timing and distances well, an early or late block will result in nearly all of your health being depleted.
Problem:
Air lifter combo.
Answer:
Let seph, lift you. This is a teleport attack. When he teleports the second time right next to you hit your retaliating slash, or the square button. Note:You must have this ability equiped. This will break sephiroth out of his combo and let you get in an air combo. This work EVERY time. Even when sephiroth is in beserk mode.
Problem:
Meteor rain.
Answer:
Run!! Jump too, it helps. Glide if you have it, but I beat him without glide.
Problem:
Wall of fire.
Answer:
Run away, if you're too close when he starts it then hit the guard key. This won't block it by any means, but you have a chance of being knocked away from the wall of flames instead of into it.
Problem:
Dark orbs.
Answer:
reflega
Problem:
He goes beserk
Answer:
When sephiroth is beserk, blocking and countering his grounded combo won't work anymore because he teleports in behind you mid combo. When he's in this state stay away from him and wait for either the heartless angel, the reaction command, or a lifter attack. The reaction block always gives you a free combo if you land it, so don't miss any of them. The heartless angel if a free trinity limit if you have the MP, and the lifter is a free air combo if you can spare the health to take the initial hit. Sephiroth begins to call in dark orbs in even greater numbers now, so be sure to wait until he's called all of them before hitting your reflega. Try to use curaga as little as possible.
Problem:
He won't die!!
Answer:
Your goal should be to get sephiroth down to 2 health bars or less. Be sure to have as many elixir's on hand as possible. I have found that trinity limit takes off just over 1 of sephiroth's health bars, so 2 at this point will finish him. When he's at 2 or less health bars hit the trinity limit. Usually after this attack sephiroth either teleports away, calls down the heartless angel, or summons his wall of flames. In the event of the heartless angel you win, because you simply toss up an elixir and hit the trinity limit again. Whether he hits you or not he still dies. If you teleports away then toss another elixir and wait for an opening for your final trinity limit. If summons the wall of fire, run/jump/glide away from him, then toss up an elixir and hit the trinity
No offense, but , how do you have so much time to write all of that. It would take me a short time to write that but it is only because i am a fast writer.