SPOILERS Is this a mistranslated line? (Japanese > English)

[chez]

It doesn't make any sense when he says "Just like you." Perhaps this is not an accurate translation of the original line in Japanese? This line doesn't seem to flow with the dialogue because we can see that Cloud and Tifa are talking to each other. It looks like Sephiroth is speaking to the Cloud who is in the memory, but the sentence doesn't make much sense. I have included a picture of the Japanese and English lines side-by-side as a comparison (credit to anonymous source: amerikajin).

I don't know Japanese but looking at a few rough translations using online sources, it seems as though the authors meant for Sephiroth to convey a sourness towards Cloud... maybe something like: "It's you! ..." or maybe "You're the one who... [stabbed me in the back]." In Crisis Core, Sephiroth says "How dare you..." But unlike in Crisis Core, after Sephiroth says it, he just walks right past Cloud.

 

[ribbit]

The line is a literal translation so it isn't entirely incorrect, just slightly confusing. ごときに (gotokini) means "like" or "the same as" so my interpretation of the line is that the translator omitted "that's" from the sentence as in "(That's) just like you." or "You WOULD do something like that". However the line is kind of a one off and there isn't a lot of context beyond Cloud having just stabbed him in the back so my interpretation isn't necessarily 100% correct either haha.

 

[chez]

The line is a literal translation so it isn't entirely incorrect, just slightly confusing. ごときに (gotokini) means "like" or "the same as" so my interpretation of the line is that the translator omitted "that's" from the sentence as in "(That's) just like you." or "You WOULD do something like that". However the line is kind of a one off and there isn't a lot of context beyond Cloud having just stabbed him in the back so my interpretation isn't necessarily 100% correct either haha.

Hey, that sort of works in the context of the conversation.

Cloud: "Sorry... I didn't get there... fast enough..."
Tifa: "It's all right... Cloud."
Sephiroth: "You would do something like that..."

It's still weird, though because Sephiroth should be talking to the Cloud in the memory scene. This would make it appear that Sephiroth is responding directly to Cloud and Tifa in the Lifestream.

(edit) I received feedback from another source that another interpretation of the line could be that the phrase is trying to convey Sephiroth's bitter feeling towards Cloud (jealousy, looking down, or despising):
"It's so pitiful/pathetic I lost to someone like you..."
"Did you really think the likes of you could best me..."

 

[Dionysos]

Hey, that sort of works in the context of the conversation.

Cloud: "Sorry... I didn't get there... fast enough..."
Tifa: "It's all right... Cloud."
Sephiroth: "You would do something like that..."

It's still weird, though because Sephiroth should be talking to the Cloud in the memory scene. This would make it appear that Sephiroth is responding directly to Cloud and Tifa in the Lifestream.

(edit) I received feedback from another source that another interpretation of the line could be that the phrase is trying to convey Sephiroth's bitter feeling towards Cloud (jealousy, looking down, or despising):
"It's so pitiful/pathetic I lost to someone like you..."
"Did you really think the likes of you could best me..."

I don’t have any experience with reading or translating Japanese, so I think what ribbit says is probably our best bet for this in terms of the meaning of the Japanese line.

I think we can ignore Tifa and Cloud's lines in this context because I don't think those lines spoken in the present in the Lifestream need to be included for flashback Sephiroth's line to flow. I don't think we gain anything by including them with Sephiroth's line, and I don't think Sephiroth is responding to them. He's just emerging from Jenova's chamber and wants to make a snide remark about Cloud.

Instead, as you say he is probably being disdainful towards Cloud. This petty infantryman has attacked him and wounded him, but Sephiroth is at this point shrugging it off and thinking there is no chance that he could actually be killed by him. In the English line that we received we could imagine that he is simply saying "Ha! Just like you to think you have beaten me" or something like that. If this is his meaning then he is of the opinion that Cloud is once again thinking he is more powerful than he could ever possibly become. Sephiroth is gravely mistaken.