If it has not been mentioned, I believe the Klingons do not fight as well as a group in hand to hand combat because they are an honor obsessed culture. Honorable combat generally requires "one on one, man to man" combat. Not hundreds of warriors taking on hundreds of enemy soldiers at once. Thus they do not make a point of practicing their skills in that regard.
When engaged in combat, they try to isolate a single enemy individual and fight until they win or die. Much like the Klingon bragging about boarding the Starfleet ship and killing the Tellarite helmsman with his Bat'leth and then fighting the captain (a friend of Sisko's) to the death.