***This Guide will contain spoilers***
Getting to New Game+
New Game+ automatically unlocks after completing the game once,there isn't any real difference in the overall difficulty if there is it's very slight,you will clearly notice this after you have got a few Medkit upgrades and gain more health.
You can acquire the Katana right at the start of the game,just when the game starts on the boat there's a door you can open inside on your right,there you'll find the weapon.
Also right before the entrance to the GeoPharm Labs,if you go the the left side of this area,you should've gotten the keycard to open the door you'll find there that contains +3 Medkit.
"Kanchanaburi" has noted in the comments that "If you accidently start a 'New Game' after the other automatically starts, then you use your New Game+ and you are NOT able to start a New Game+ from the main menu"
General combat strategy
You can rush trough a good portion of the game if you want,zombies won't be able to follow you on ladders or in a space where you need to crouch,also of course with some patience by being stealthy you can easily dispose of zombies without being attacked but it's not always possible as sometime you will have to face them directly especially at the end of the game.
Avoiding running when in a fight,zombie will easily outrun you and possibly kill you and the camera automatically focusing towards where you are running won't allow you to see zombies that are chasing you,using the fast dodge by pressing B it's the best option you have at avoiding zombie's hits,pay attention to your stamina and try to dodge when the enemy is just about to hit you to fully take advantage from the dodge action.
The most efficient way and quickest way to kill a normal zombie (this does not include special zombies or these that hold a keycard or "The Butcher" cannot be knocked down) is,while you are running towards them by pressing B and then using RB you can kick them and knock them down,this allows you to 1 hit kill them by pressing A to perform an "execution".
Avoid doing this when there are different zombies close to each other as they could kill you while performing the "execution" move.
The Butcher enemy
The most annoying enemy,at least for me,was "The Butcher",what worked for me was,if i had enough ammo for the shotgun (10 are enough) i would use it to kill him,when he gets close to you quickly shoot him when he isn't blocking with his claws,if you do it at the right time the shot will stagger him and by spamming multiple shots he will be infinitely staggered until' he dies or you run out of ammo and need to reload.
If you do not have a shotgun you need to fight him melee as the pistol is useless,of course try not to run out of stamina as if you do he will just chain attack you and you'll die quickly without any time or possibility to dodge his attacks.
What i did was target him (LB button) and wait for him to finish his combos and quickly hit him once or twice then back off,i basically repeated doing this until' he was dead.
Zombie Linda
Linda at the end of the game could also be a bit annoying,if you have the shotgun if you gain some distance from the other zombies you can just infinitely stagger her the same as with "The Butcher" enemy and kill her easily.
If not killing her melee the same way as with "The Butcher" described above works too,in this occasion though you need to gain some distance from the other zombies before trying to hit her.
Don't bother fighting "The Bouncer" and "The Butcher" on the platform at the end of the game as if you stay close to the edge they will often jump towards you only to fall off and die without you doing anything.
If you vote negative a comment would be appreciated so that i can improve the guide otherwise it wouldn't be of any help,thanks.