There was one example, in mission 38 I believe it was, I beat the thing as fast as I possibly could but got no stars at all. But I looked online and saw that you had to use sneak smoke (the sol where it makes you less detectable) to get a preemptive attack and I ended up 5 stars. So it's very possible you have to get the strategy they intended to get the 5 stars.
I think you're referring to Mission 35 with the armored enemy.
As far as I know, getting 5 Stars is time related. However, if you finish with the target time, you'll only get 3 stars so you'll actually have to finish the battle in a significantly less time. About 10seconds or so less than the target.
Did you level all the weapons you picked up r just the main ones?
Well i'll answer both:
If you did it evenly to all the weapons you picked up, prolly a bad use of supplies and money, like i said, there are better uses for money and save them for individual items. Also, leveling weapons, while helpful, isn't completely necessary, you should be fine
If you stuck with the same weapons you should be fine. I am not sure if you can "overlevel"
My only real recommendation though is to worry about your weapons unless you get stuck. But level as you feel necessary.
At chapter eleven i had only transformed Lightning, Fang's, and Vanille weapon once. I didn't need to, but any help is good help i guess.
I am not one to grind levels if i don't have to tho
is it ok if I overleveled my other weapons? I'm currently at chapter 10.
Depends on whether you care about your battle ratings or not. Your target time (and therefore star level) for fights depends on your stats. This is not relevant unless you are trying for a platinum trophy. If you do not care about trophies, then I don't see why it would hurt you.
Do a little level grinding, starting with flans, or goblins. AVOID BEHEMOTH KINGS! Continue into the story a bit, and the rest should take care of itself. i did very little grinding and maxed out my crystarium for the main roles before chapter 12.
One more thing...try buffs and debuff if you are not already. Sazh's haste is a lifesaver.