when saying "strength", tobirama isnt usually associated with it. However, he is tactful, extremely fast, analytical, has above average powerful jutsus, and years of fighting in real wars. His fighting is just like minato's- utilization of the hiraishin jutsu and acting before the enemy does. you can probably gang up on him with a few kakashi/gai level shinobi but 1vs1 is extremely deadly... you know the hidden stone declared a flee on sight order when they saw the yellow flash? Tobirama is at that level and his fierce chakra makes him a tad bit above that i'd reckon
EDIT: only reason orochimaru could bind tobirama was because he was using hashirama's chakra. if not, tobirama would have broken out of the jutsu and probably have killed someone
when oro was controlling the hokages, minato and hiruzen could barely move about. i dont know if that was due to oro's binding ornot but tobirama had enough freedom to move about and build up a huge amount of chakra.
same goes for hashirama. suigetsu literally melted. and this suigetsu was doing very well vs kisame and had the confidence to go toe to toe with him