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This is just me, but he probably has some type of hidden strength within him.

You have to realize that there was a reason that he was chosen to become a Sephiroth clone. Cloud, and Shinra most of all never saw Cloud's potential until the defeat of Sephiroth in the Mt. Nibel Reactor. But he always had it in him to be the strongest soldier.

Personally, I don't see how he never made it into Soldier. I figured he would have been the top of the class.

I think that it was right that it was an adrenaline rush.

You guys have to understand, there was a reason Cloud didn't make it into SOLDIER initially, he wasn't good enough at first, so, ashamed, he became part of the private army.
Now, consider this, Sephiroth just laid waste, absolute WASTE of his villiage, killed his town (and probably his mother if I remember correctly), killing Tifa's dad and almost killing Tifa HERSELF. Cloud is... mad. Furious, desperate, on the verge of death himself, and THAT'S when his potential power starts to awaken. At that point, an assload of his ability was brought to bear as Hojo tried to use him to replicate Sephiroth.

The only reason Cloud wasnt in SOLDIER was because he had a negative reaction to Mako. This negative reaction, which results in a comatose-like state, is also showcased during his escape from Shinra Mansion with Zack and after he is found in Mideel in the game.

That negative reaction failed him. He was a perfect candidate other than that, so his strength mixed with the pure emotion and adrenaline from his whole life being stripped away from him before his very eyes, lead to his being able to defeat Sephiroth in the reactor.

The Jenova cells had little effect on him. It was the doses of Mako that did him in. Thats why in the testing for SOLDIER he was ejected, because all of them receive a mako shower to increase power. Cloud had a negative reaction to it.
And later, in the Shinra mansion, the two were kept in Mako filled tubes, thus resulting in Clouds comatose state there.
And in the Lifestream, which is the purest form of Mako, he also went under the same effect.

Jenova cells were only present in one of those occasions, so cannot be attributed to all three.

Soad wins the internet.

In terms of SOLDIER standards Zack is probably more skilled than Cloud in most regards. To be honest Cloud isn't anything special, if he didn't have his MAKO shower boost and JENOVA Cells he would be utterly useless, just like the run of the mill ShinRa grunts you encounter throughout the game. Like he was trained to be.

The only reason Cloud defeated Sephiroth is because he went in there with the intention to kill him and get whatever revenge he could in an adrenaline rush/fit of rage, if you ever see someone completely and utterly flip their lid in reality and start fighting it's a similar thing, they just don't give a crap, they just keep tearing at the person, adrenaline rushes dull pain to the body which would explain why he just picks Sephiroth up by his sword and throws him off a bridge.

Zack, give him his dues as a great character, only went in there with the intention of saving the day because he knew Sephiroth was in the wrong in my opinion. I don't think there was any real emotional attatchment with his fight apart from maybe a sense of betrayal from Sephiroth, and if we take into account the OVA which shows the fight between them really the only thing that saves him is his sharp nerve and his fighting skills, in which Sephiroth is above him entirely, resulting in his loss in that fight.

Cloud didn't use any SOLDIER training, or hell even grunt training, he just got pissed off and skewered Sephiroth like he deserved. xd Cloud is an impulse fighter, the points when things go hay-wire for him emotionally is when he really decimates things and overall probably displays his fighting prowess then, not in skills but by how easily he can destroy things when he's pissed. Zack relies on skills, if he had gone in there as angry as Cloud had then he probably would've won.

Then again you could just undermine everything that's been said and state that Cloud caught Sephiroth off-guard, which is pretty plausible seeing as Sephiroth makes no effort to attack him until it's too late.

I'm aware that the previous mentioned notes are correct but that still isn't stopping me from making a little note: xp

He had a pre-emptive materia equipped. A reason why he was able to sneak up on Sephiroth and stab him while the latter was in his mantra.