Those random chests you can't open if you want the best weapon in FFXII has to be one of the stupidest game design decisions of all-time. I never look at walkthroughs for RPGs until I'm at the end game and when I read that part I was like "fuck you Square!". I didn't restart my game for the weapon though.
I did the same but I never did guides back then anyway, only after I completed it. I mean in that game I spent most the time leveling up all my characters, it wasn't until the Boss before the Veira village, the wood wurm that I realised I couldn't do that, I needed to level just 3 people to actually win the game. Else I'd be doing nothing but farming XP.