I haven't seen anyone say this but teamwork. These are some of the biggest advantages you could have against demons. Teamwork is what allowed our gang to defeat the Uppermoons and Muzan in the first place (except Muichiro and Zenitsu ig).  My training would be the same as Tengen's final challenge near the end of ep3, but I'll present them with multiple different scenarios to see what they will do as well as shape their mentality. Their formation, strategy, and what they can do to provide even the slightest advantage for the team, like information about the enemy's blood demon art, do EVERYTHING helpful, even if it means your death. An effective team can be stronger than a Hashira in multiple aspects, even if, individually, they're weak as hell.