At the start of the level, head right and search the room, there are usually some useful items that will spawn here. Head on to the garage as there are usually tier 2 weapons and even more molotovs or pipe bombs around.
After you activate the crescendo, you can hide in the room that is next to the gas station. It is pretty safe and easy to defend as you wait for the crescendo to end. If you desire, you can actually run all the way back to the saferoom as it is nearby.
Alternatively, if you have a grenade launcher, you can actually destroy the shutter door and skip the crescendo.
Personally, I'll usually run and ignore the crescendo. There is considerably risk involved as a tank may spawn ahead. Once you have entered the building, jump down to the second floor, grab the pills from the first-aid cabinet and carry on running all the way through the map. If you do this, you won't encounter many zombies further ahead in the map as they have all spawned far behind you.
If you did not wait for the crescendo to end, quickly check the rooms on the second floor for pipe bombs or molotovs then jump down to the first floor and continue running, don't take the stairs as you need to backtrack.
If you have waited out for the horde to end, then you may want to search the rooms on the first floor as they have a chance of spawning med-kits. It is useful to activate the shutter that is shown in the picture above, as it'll provide you a chance to run away into the open if a tank spawns ahead.
If you did not wait for the crescendo to end, jump down the first manhole as the room on the right has a chance of spawning zombies. If you waited for the crescendo to end, search the room on the right for supplies before dropping down the manhole.
There is a decent chance that a tank may spawn here. So throw a molotov and kite it with the aid of the small tunnels. Do not allow the tank to walk over the water as it will extinguish the flames.
Try to ensure that everyone climbs up the ladder at the same time as there is normally a sizable horde at the top. Just to be safe, I'll usually save a pipe bomb for this section. Remember to check the ambulance for med-kits before entering the saferoom.