Right, lemme clear this up, any non-MS genjutsu technique that hits Tobirama will easily be dispelled by using genjutsu dissipation; 'The ninja needs to stop the flow of chakra in their body, and then apply an even stronger power to disrupt the flow of the caster's chakra; this is called Genjutsu Dissipation (幻術解, Genjutsu Kai).' If not that, Tobirama can sense a build-up of chakra since a build-up of chakra is required to activate any ninjutsu/genjutsu. Once he's sensed that, he'll be on the move so Amaterasu isn't going to touch Tobirama. Another counter to genjutsu is kage bunshin.
Hiraishin seals aren't physical things, damn, this concept needs to be grasped by most members; Juubito used 4 Juubidamas and decimated an entire area and Tobirama was still able to Hiraishin to those destroyed areas. While Itachi is busy trying to create an Amaterasu ring, Tobirama can employ kage bunshin to take him out. Itachi has to be stationary and focusing on different areas to create a ring made of Amaterasu and even if Itachi tries that, he'll be extremely exhausted and would then pull out Susanoo, something Itachi can maintain for around 5 minutes. Tobirama can easily outlast Itachi in Susanoo, and then Itachi dies due to chakra exhaustion.