The strength of a spell junction is directly proportional to the number of uses of that spell a character carries - so draw, draw, draw! The maximum uses a character can carry of a spell is 100; try to have that many of any spell junctioned to a statistic. It's well worth the time spent repeatedly drawing.
Also, when your character falls into the critical hit-point zone, hit the triangle button continually to skip turns until his limit break becomes available. Don't waste his hurt status on a regular attack!
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source:
here
