Use a machine gun ofc, but use the machine gun like Vityaz, G-21 C, or UMP 45. those guns.. I think they're like the best on the game (In my opinion)

For Pistols, I suggest using revolver, they do big damages. You can use G-17 too, thats a gud gun.

When playing stories:

Revive: make 1 person a bait to the zombies, and heal the wounded one as fast as possible. If possible, make that peoples that have medic kits revive peoples.

Medkit tips: use the medkit when your health is on 50%, cuz in hard, health is like one of the most important things you must save.