Don't worry. You can get the Killer Bow in Lhusu easy again, just keep going to that same chest over and over, zone 3 screens away and come back and try again, that works for all chests, though you won't always get the same thing.
Hmm, a bit underleveled, but for now, I suggest focusing on one team of 3 for the majority of the game, so that team is always prepared. This means your other 3 characters rot in the menus, but it's a bit easier this way for a normal game sometimes. I suggest training a lot though, however dull that may sound lol, getting to level 16 by the end of this place is my minimun recommendation.
So you opened the Zodiac Spear "no-open" chests, huh? XD No worries, there's another way to get it if you're dedicated. :P Make sure you have quickenings, at least one per character, and upgrade weapons as much as you can. An awesome place to train is the Nam-Yensa Sandsea at this point, not on the Urutan-Yensa, but on the Bagolies in the northern areas. Use Cypress Poles to make quick work of them, which you can get from the Bazaar, and also a chest on the railings in the furthest area of the Nam-Yensa, the Simoon Bluff. Make sure you have Cura as well, since Cure could be getting a bit medeocre at this point. =)