Tobirama can just ignore Naruto's clones, and instead go straight for Susanoo and teleport it away from Itachi leaving him vulnerable to a blitz from either one. Both of them can react to Naruto and his shadow clones, so getting up close to Susanoo should be no problem. If Naruto tries forming a ring of clones around Susanoo in a sort of "blockade" to prevent Tobirama from getting close, Gai just uses Hirudora and kills all the clones.
As for the AOE of COFRS, Gai or a shadow clone can carry a marking far away enough so they can teleport to escape its aoe. And there's also the fact that Naruto can't explode it too near himself or he gets caught in the explosion himself.