Listen in the old comix, Master Splinter explained how the bo staff was in itself a defensive weapon capable of disarming any adversary. Plus a lot of people sleep on Donatello's skills. He is not the weakest one, he is the one with the least aggression. Therefore he will not fight with as much intensity in a fight as the rest. Now bloodlusted, Donny is well capable of taking out either Raph or Leo