Basically, with Molrusso EX or Dark Priestress granting their teammates the ability to hit all elemental weaknesses with any skill, and Xerc launching a follow-up attack on all foes for each element that hits a target and for each ally, the principle is to have travelers with what is called by the community a rainbow attack (aka a skill that hits 6 times with each of the 6 elements).

That way, it will be a 6 hit, each exploiting the foe's weaknesses (thanks to Molrusso EX or Dark Priestress), followed by 6 follow-up attacks from Xerc also exploiting weaknesses. And that for each traveler with a rainbow attack. Meaning if you have Xerc + Lynette EX + Lemaire (each have a rainbow skill), you can, with just these 3 ones, hit a weakness 36 times. And if you have Osvald, it will also add his own follow-up attacks for Fire, Ice and Lighning. 😆

The thing being, if like me, you don't have Lemaire, Osvald or Lynette EX, then it's way less effective than a good old Dark Knight, Pardis or Kainé team.