After level 4 make sure you throw a grenade at the last Zombie when it is climbing through a window, so it crawls.

This would give you time to board the windows and gather decent weapons.

Do this near the end of every level. After the 4th level go into the basement room the one with the mystery box.

Level 8, go up the stairs in the basement and go to the grenade section on the wall.

Set your sensitivity to very high or insane and make sure your friend is with you and has got a Ray Gun or Browning or MG 42.

Whilst your friend shoots, you turn to the grenade section and keep on throwing grenades at the doorway in front of you.
Keep on doing this and make sure your friend tells you when they have to reload otherwise you're dead!