Final Forms

Sephiroth does not have a physical form, since his physical body had been damaged after Cloud threw him into the Mako Pit, so all the Sephiroth forms seen here are illusions that he had created through JENOVA. Each form that he wields seem to imply a stage that he is going through on the path to Godhood. Sephiroth does not actually change form: only those injected with monster serum are physically affected and transformed; no other test subject that had been injected with JENOVA cells (save for Vincent, but that was because he was also injected with monster serum) changed physically, although they can be controlled and altered mentally by Sephiroth and JENOVA.

Bizarro Sephiroth

Appearance: bloated Sephiroth resembling a winged larva
Form: First
Song Theme: Birth of a God
HP / MP:
- Bizarro Sephiroth: 40000
- Head: 2000 / 400
- Right Arm: 4000 / 400
- Left Arm: 4000 / 400
- Core: 10000 / 400
Item Drops: Nothing
Attacks to Watch Out for: Bizzaro Energy (Sephiroth heals self with 6500 HP)
Difficulty Level: Medium
Recommended: - Double Cut materia mastered
- Switch party as soon as prompted; switch party again when prompted to be able to attack the Core
- Fast win: Attack only the body. Sephiroth can heal up to 6500 HP though, so make sure damage inflicted on him is more than that
- Limit breaks and Summons

Other things of note

Sephiroth looks really ugly in this one, but this is a visual imagery of his 'transformation'. Just as a larva bursts out of his cocoon to eventually become a butterfly, Sephiroth is on the verge of coming out of his own chrysalis, to become even more powerful by the next form. It can be physical manifestation of Sephiroth's belief that he is about to become a God, rather than actually becoming a God right then and there, and this is the mental image that he projects to Cloud and the party.

Safer Sephiroth

Appearance: Four moving wings replacing Sephiroth's legs and feet, right arm is replaced by a purple-black wing; everything else is Sephiroth's human form
Form: Second
Song Theme: One Winged Angel
HP / MP: Unknown
Item Drops: Nothing
Attacks to Watch Out for: Super Nova (illusion of a meteor smashing down on earth - HUGE damage)
Difficulty Level: Difficult
Recommended: - Double Cut materia mastered
- Equip Ribbons for status protection
- Quadra magic spells
- Limit breaks and Summons
- Lots of healing items
- Anything else you have handy; this is hte last battle, so bring out everything you have!

Other things of note

The "emerging as a butterfly" aspect after the previous Larvae form Sephiroth uses. Again, Sephiroth uses his mental abilities to depict himself to Cloud and party as a God who has risen from his coccoon to kill them all - it's impressive looking, as is his Super Nova attack, and unless there's a lot of Heal / healing items, it's very easy to be killed.

Final Sephiroth

Appearance: Normal Sephiroth in human form
Form: Third
Song Theme: none
HP / MP: N/A
Item Drops: Nothing
Attacks to Watch Out for: None
Difficulty Level: Stupid Easy
- Whether or not you have Omnislash as a final limit break, you'll get it here. One slash, and the battle is over.

Other things of note

I don't think that this final battle with Sephiroth implies that Sephiroth himself is in actuality, weak, but take note that the last two forms he had must have used up a vast majority of his strength. Never one to quit though, Sephiroth wants one final battle where he tried to control Cloud with his JENOVA cells, and loses: Cloud is far far stronger than he had been since their last meeting, and Sephiroth underestimated him again. This is actually a contest of wills between Cloud and Sephiroth (this is why CLoud can use his Omnislash, even if you haven't gotten it as a Final Limit Break yet); it proves that his mind is now stronger, now that his memories are secure, and is able to fight off Sephiroth's influence. In the animation that follows after beating Sephiroth, you can see that both Sephiroth and Cloud are purged off the JENOVA cells, and Sephiroth seems to implode.