very difficult, the zombies appear in front and behind without a chance... The aim is slow and imprecise.

I beat the demo without healing (I couldn't figure out how to), here is my advice. 

For the first level, I only used the pistol. It's got great accuracy, kills most zombies in one hit, reloads almost instantly, and you'll get plenty of ammo for it. This also saves up my other ammo for level 2, which I almost entirely needed.

Rely on your speed to keep the zombies away from you, don't ever let anything reach melee range. Always use headshots, the pistol makes it easy. Constantly check behind you, the game will spawn zombies where you've already been, you gotta spot them coming or they'll swarm you. Spend as little time in buildings as possible, they're death traps. Kill everything first, then loot buildings ASAP and get back outside, you're at your least vulnerable  in wide open spaces. 

For the second level (the boss fight), prioritize killing the mobs first. I used the magnum here just so I could kill on bodyshot more reliably. Keep moving, get everyone chasing you in a clump and then kite them around the map, weaving through map objects so they get stuck on them and slowed down. Prioritize killing sniper zombies, then fast zombies, then regular zombies, saving the boss for last. Once the boss is alone, I switched to AK47 because it dealt steady reliable damage, swapping to the carbine after I ran out of AK ammo. Keep kiting and getting the boss stuck on objects, you shouldn't have to worry about getting hit once it's the only thing left on the field.