Help with some phrases
 

Hello, I need help with some phrases. Any help is greatly appreciated :)

1) １コしか違わねークセに高校生だからっていつまでも年 上面しやがって
“We’re not very different, but just because she’s a high school student she’s always going to act like she’s older.” ?

2) アイツさ後輩のクセに言うことなんか説得力あんだよな ー
“She’s a junior, yet what she says is kind of persuasive.” ?

3) すっげー美人て超ウワサじゃん前から１回お手合わせ願 いたくってさー
“It’s a huge rumour that she’s a real beauty. For a while I’ve wanted to play a game with her.” ?

4) それはその頃あたしたちの中で流行した遊びだった
“At the time, that was popular play among us.” ?

5) もう少し　こう顔と髪をいじって似せて
I don’t understand “こう” and “いじって似せて”

6) 部活ないのをいいことにせりを襲っ。。。襲うとは！
I don’t understand this at all.

7) せりにはせりの良さがあんだから
“Seri has her own merits” ?

8) 少しアピッといてあげるか。。。
What does“アピッといてあげる” mean?

9) あとはうまくやりなよセリ！
What does “うまくやりな” mean? Does it mean “Be successful” ?

10) 生史くんをオトしちまいな！
What does “オトしちまいな” mean ?

11) あたしは亘でも誘って出掛けるか
“I guess I’ll invite out Kou or something.” ?

12) 恋なんかするだけムダで誰からも必要とされない人間な んだ
“There’s no point in falling in love. I’m a person who isn’t needed by anyone.” ?

13) 中学生のうちはチューまでだからな
“You can only go as far as kissing at a junior high student’s house” ?

14) 生史に幸せにしてもらえよ
“Make [her] happy” ?

1) １コしか違わねークセに高校生だからっていつまでも年 上面しやがって
“We’re not very different, but just because she’s a high school student she’s always going to act like she’s older.” ?

Nago: But she IS older if only by a year. １コ in colloquial Japanese means "one grade" or "one year".

2) アイツさ後輩のクセに言うことなんか説得力あんだよな ー
“She’s a junior, yet what she says is kind of persuasive.” ?

Nago: Precisely!

3) すっげー美人て超ウワサじゃん前から１回お手合わせ願 いたくってさー
“It’s a huge rumour that she’s a real beauty. For a while I’ve wanted to play a game with her.” ?

Nago: You've got it. (Only from the Japanese, there's no mention of a game, and this sentence CAN mean something entirely different. [something I can't say on JF]. Just rememeber not to say to a man "I wanted to 前から１回お手合わせ with you!". )

4) それはその頃あたしたちの中で流行した遊びだった
“At the time, that was popular play among us.” ?

Nago: Correct.

5) もう少し　こう顔と髪をいじって似せて
I don’t understand “こう” and “いじって似せて”

Nago: こう here means "Like this". いじって似せて means "do things (to the hair) so you will look like someone. "

6) 部活ないのをいいことにせりを襲っ。。。襲うとは！
I don’t understand this at all.

Nago: It means "Only because (he) had no extra-curricular activity that day, he attacked Seri." At the very end, something like "How terrible!" is left unsaid.

7) せりにはせりの良さがあんだから
“Seri has her own merits” ?

Nago: That is correct. あんだから is the Kanto colloquial speech for あるのだから.

8) 少しアピッといてあげるか。。。
What does“アピッといてあげる” mean?

Nago: Lol. It's from your language! アピる = to appeal to someone. 少しアピッといてあげるか means "I will do something so that you will have some appeal (for someone)."

9) あとはうまくやりなよセリ！
What does “うまくやりな” mean? Does it mean “Be successful” ?

Nago: Yes. "Do things right."

10) 生史くんをオトしちまいな！
What does “オトしちまいな” mean ?

Nago: It means "Make him yours!" オトしちまいな in the dictionary form would be オトしてしまいなさい.　オトす comes from 落とす, but since it's being used for a non-dictionary meaning, it's written in katakana. 　

11) あたしは亘でも誘って出掛けるか
“I guess I’ll invite out Kou or something.” ?

Nago: Yes. It's like saying "Why don't I try asking him out?"

12) 恋なんかするだけムダで誰からも必要とされない人間な んだ
“There’s no point in falling in love. I’m a person who isn’t needed by anyone.” ?

Nago: In essence, yes.

13) 中学生のうちはチューまでだからな
“You can only go as far as kissing at a junior high student’s house” ?

Nago: No! (I can finally say 'no'. You're too good.) This うち doesn't mean "a house". It means "as long as (one is a junior high school student)".

14) 生史に幸せにしてもらえよ
“Make [her] happy” ?

Nago: Nope. "Let 生史 make you happy."

Thank you very much, Nagoyankee :) I just have two more questions.

This confuses me now, lol. What does “いつまでも年 上面しやがって” mean? I thought that “面” meant to act or pretend.

So, does ~をいいことに mean “Only because of X they did Y”? Is it like, taking advantage of a certain circumstance and doing something?

面, all by itself, doesn't mean to act or to pretend. It means "face".
Yet, when it's in the form "Noun + 面 + （を） + する", it means "to overly act like (Noun)" or "to excessively emphasize the fact that one is (Noun)".

What is clear from the sentence is that the speaker is a 9th grader (last year of junior high school) and the person being talked about is a 10th grader (HS freshman).

So, the guy is actually older by one school grade (quite possibly only by a few months or even less). But the speaker doesn't like the way that HS freshman uses that chronological fact to order him around.

いつまでも年 上面しやがって = You have (or he has) been mis-using your "senpai" advantage too long!

Exactly. It's one of the phrases we native speakers use very often yet Japanese learners seldom do. In other words, it's something that will help you sound natural.

Awesome, I get it now. Thank you very much, Nagoyankee! I really appreciate your help :)