This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Wrexsoul is a clever one. He only shows up when a randomly selected
character in your party is either near-fatal or wounded.
Wrexsoul chooses a pattern of possessing your characters before the
fight begins, and sticks to this pattern throughout the fight.
To get him to appear, you must hurt or wound a character, and see if
Wrexsoul appears. If not, rejuvenate the person and try on another
character. Keep going until you've figured out Wrexsoul's pattern so you
can easily beat on a character to make Wrexsoul appear and then beat on
Wrexsoul in the little time you have before he disappears again.
Desperate? Read on...
Of course, you can always cast X-Zone on the SoulSavers. If you can get
both SoulSavers into the X-Zone, you've automatically won the fight.
Careful! If you suck the SoulSavers into the X-Zone, you won't get the
Minerva that Wrexsoul drops if he was defeated the usual way.