The spell is only used when the Imp's horn has been broken. It will cease using Abrasive Tantara and Deafening Tantara and only use Frenetic Rip. Once it has used Frenetic Rip, it will regain its horn and resume its regular behavior. Note that it is possible to break the horn and have the Imp regain it before it uses Frenetic Rip.
The horn is broken due to critical hits. The side on which melee attackers are facing appears to have no effect.
The Imp's level against the Blue Mage and/or party members greatly affects the chances of breaking the horn. It is far more likely to break the horn of a lower level imp than a higher level one relative to the level of the melee attackers.
High level (ideally 75) jobs with innately high critical hit rates and/or weapon skills with critical damage are recommended.
Breaking the horn is a rare occurrence, regardless of preparation.