Apparently he was killed by two kyubi coated monsters and not knowing any sealing jutsus was his downfall since he had nothing in his arsenal that could put kyubi monsters down. Assuming he had no strong edos either. Plus kishi didn't plan to f**k power scaling when this info was released.
Guess he was overwhelmed by the platoon. It was said to be the best in...was it Kumo? Yeah I think it was Kumo. Hell if Madara hadn't used sunano'o while facing Alliance he might have met a similar fate.
It was the top 20 jonin of Kumogakure and he was ambushed.
The data book has Kumo with the highest star rating in terms of military power out of all the villages.
Besides that Tobirama was fighting during the war so its not as if he was completely on top of his game and well rested.
He must of been fighting for days at the least by the time the Unit was assigned to take his team down.