Defeat an enemy, use Life spell/Life1 item on defeated enemy BEFORE they disappear, and they'll revive again. However, they will revive with 0 HP, and will thus die immediately again. You will not see them revive nor hear them die again, but it will be apparent in the EXP/GP screen.
Note, this is done on the SNES version. Whether or not this works on any other version is unknown to me.
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The reviving enemies glitch is a glitch where defeated enemies are revived. Usually defeated...