Ice assuming you have equally leveled up all 6 atributes, edo hiruzen benefits exactly the same way as other ninjas, not more not less. Depending on a ammount of elements in a mystery, each mystery has its own multiplicative factor.

For example sage naruto with wind atribute gets value of "1", while edo hiruzen with 5 elements get "0.2". So if you have 10% bonus on each of 6 types, mentioned naruto gets 10% wind bonus * 1 = 10% increased dmg. Meanwhile hiruzen has 5(elements) * 10% * 0.2 which gives exactly the same 10%.

If mystery is non elemental (like susanoo sasuke, it gets multiplicator of "1" and takes normal att stat as boosting element).

This all is just to point out, that every ninja benefits exactly the same from this feature (as long as its evenly upgraded) and it doesn't favour multielemental ninjas.

source: https://forum-narutoen.oasgames.com/page/show-post-37039-1.html, last paragraph.

Actually SSirus can be right, since towards fukurokumaru's deal counts every ingot spent anywhere, but in some places used coupons dont count (there's a list of things in event description which features count, so its safe to assume others dont). There's no way to check it yet, since there hasnt been fukurokumaru event since that system's released yet.