Big combat not mass combat can drag on and give no challenge to the party. One method I have been using is if you have 40 Zombies attacking the party break it down into large swarms. for every 10-15 creatures create one huge swarm, use the swarm rules such as auto damage for the base attack weak to AOE attacks, resist slashes, times hit points by exchange ration eg X10 etc. the characters will mow down these very fast and you don't need to the book huge keeping. but because the swarms are doing auto damage they will still worry the party.
I have done this with Zombie, Goblins, Ice Mephits and Skeletons and it works fine. Player really freak out when they see a couple of swarms approaching.
cheers
Mark