My guess is that the chance to break horns (or weapons/tusks/etc, on any ToAU breakable mob) has something to do with your level vs theirs. I've only done a few imp exp parties because I hate their TP moves, but I've rarely seen the horn get broken during them.
I got Frenetic Rip with my wife on her 75 RDM. My crits/WSs/spells rarely broke horns, but she had an easier time of it, even when I was using sneak attack. When you were the one helping someone else get the spell, were you on a 75 job?
Just a guess from experience; no testing done, but either way I would suggest enlisting a lv75 1-handed DD job or MNK to assist. If you can't, you might be better off fighting the imps on the whitegate/nashmau boat. They only appear at night, but are lower level mobs.
This post has been edited by Raldo: 17 May 2010 - 12:05 PM