I highly recommend to combine this with the Materia Mastermind and/or 99.999.999Gil achievements.
The easiest way to obtain this is defeating the Magic Pot in the Northern Crater.
You have to feed him an Elixir and then defeat him for a whooping 8000XP and 1000AP. Some times they come in two.
If you buy the EXP Plus Materia for 2000GP in the Gold Saucer you can expand the EXP outcome to 12000XP or double it up to 16000XP.
Sometimes there is a guy on this spot at the Gold Saucer.
If not just leave and come back again. Here you can buy up to 100GP with 10.000Gil. If you rack up the 2.000GP you need 200.000GIL.
You also need the W-Item Materia you found this in the Sector 8 Tunnel raid "on Disc 2". You have to go the tunnel all the way to the south even if characters say this is wrong. Alternatevily you can dig this later out in Bone Village. You can duplicate as many Elixir as you like, if you have a minimum of two. With W-Item equipped take one Elixir. Take another and with
to "stop" using it push
again and so on. Every time you duplicate one up to 99.
Here is a video solution for this.
(Video credit goes to 19HajimeSaitou91)
Level 4 Limits are not needed and also you do not have to fight alone.