I think I prefer 2nd Editions Frenzy rules to be honest. It has more of a handicap and seems to emulate Anglo-Saxons going battle crazy at Hastings and similar - forced charges, failing to contact and routing are all the costs paid for getting the combat bonuses, but still drive narrative events of the battle and give strategic choices (trying to control the unruly sods!). In 3rd it seems like there's no downsides to Frenzy, which make it less interesting.
There's no cost to Frenzy, so you're not losing any opportunities by taking Witch elves over Warriors apart from the save pip from shields. Anyhow, if you want to go Frenzy, seems like problem is that the cold, aloof Elves have such a high Cool, and will only go into Frenzy about 30% of the time wheras the humans 70%). A leader model can whip them into a frenzy by adding their bonus to the die roll rather than the stat, a magic ring can have 'cause frenzy' embedded on it (but then using it on hardier troops might more sense).
Alternatively use a tactical ploy - have your Warriors engage a unit, and have the Witches repeatedly charging then withdrawing from combat against the same unit - each charge gets 30% of going into Frenzy, and the high Leadership makes it unlikely they'd rout.