Mondatime

I didn't know much about this (RNG manipulation in FFXII) until recently, which is a shame, because it's a great exploit! It can be used to farm rare items in chests.

In a previous game, I spent hours to collect just a Ribbon (or maybe multiple Ribbons) from a chest. Poo. >=< This would have been nice to know back then.

Anyway, Necronopticous and I have been abusing this exploit a little in our challenge. We acquired two Zodiac Escutcheon shields from a chest in the Barheim Passage, and -- from the Cerobbi Steppe -- pouches of Stone Shot and Dark Shot. Stone Shot has replaced Onion Shot as our main ammunition, and Dark Shot helped us speed through the battle with Ultima. =D

Basically, you just remove your weapon, set a Gambit of Self -> Attack, and search for a pattern in your hit combos before collecting the chest of your aim.

a) 3 hit combo after 23 hits - Continue - 4 hit combo after 29 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon

b) 4 hit combo after 56 hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.

c) 5 hit combo after 82-86 hits - Open chest and get Zodiac Escutcheon.

d) 2 hit combo after 26 hits - Cast Curaga - 4 hit combo after 27 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.

e) 3 hit combo after 33-36 hits - Cast Curaga TWICE - 5 hit combo after 48 more hits - Open chest and get Zodiac Escutcheon.

f) 2 hit combo after 21 hits - Cast Curaga - 3 hit combo after 41 more hits - Cast Curaga - 4 hit combo after 7 hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.

g) 2 hit combo after 18 hits - Cast Curaga - 4 hit combo after 56 hits - Continue - 5 hit combo after 28 hits - Open chest and get Zodiac Escutcheon.

h) 2 hit combo after 18 hits - Cast Curaga - 3 hit combo after 21-23 hits - Continue - 4 hit combo after 29 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.

i) 2 hit combo after 18-20 hits - Cast Curaga - 3 hit combo after 21-23 hits - Continue - 2 hit combo after 37 more hits - Cast Curaga - 2 hit combo after 9 more hits - Open chest and get Zodiac Escutcheon.

j) 4 hit combo after 4 hits - Continue - 2 hit combo after 10 more hits - Cast Curaga - 3 hit combo after 41 more hits - Cast Curaga - 4 hit combo after 7 more hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.

k) 3 hit combo after 56-63 hits (approx) - Cast Curaga - 4 hit combo after 7 more hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.

Source: Getting the Zodiac Escutcheon from Barheim Passage- Guaranteed - Final Fantasy XII - Neoseeker Forums

Note that Grand Master's guide, linked above, works on other chests, too, not just the one specified in his guide. We've used it to acquire items that have only a 10% chance of being collected in three different chests.

Also, relating to the specific chest in his guide: we found that it almost always appears if you attack yourself 15 times before entering the same screen as the chest. It's much harder to find a pattern, though.