This is my english translation of this guide http://steamproxy.net/sharedfiles/filedetails/?id=271751703 (in french).

Thanks to Flow for his help for the English translation.

» Use :
The Medic's main objective is to heal players and to tank large zeds (Scrake, Fleshpound, Patriarch).
To put it shortly, the Medic attracts zeds and get hit for his fellow teammates, while crouching to allow them to shoot the baddies, since he has an enhanced armor.
The Medic can use the syringe but also the secondary fire of the mediguns to heal his teammates.

» Perks :
The Medic is faster.
He can heal way more efficiently than any other class (obviously) and his nades now make a green smoke that provide a healing area as well as hurting zeds within it's AoE.
His armor is much more resilient and far less expensive than for other classes.
Medic guns are more powerful at high level against zeds, and their cost is also reduced. The reloading time is also shorter, combined with larger mags.
He can't kill himself with his own grenades (since they heal ^^).
PS : to level up, you need to heal teammates.

» Spawn :
Armor (lvl 5)
Armor + Medigun MP7M (lvl 6)

» Best weapons :
Medigun M7A3 : large mags, powerful, and the crosshair changes color when the mag is almost empty (use mostly against smaller zeds (clots, etc..)
Medigun Schneidzekk : Very powerful weapon, but smaller mag, not very precise. Use it against larger zeds.
Medigun MP5M : Not so powerful, but the perks are a great precision, high fire rate, large mags. Use it against smaller zeds.

» Use :
This class is meant to eliminate every type of zeds, big and small, especially if they are tightly packed.

» Perks :
The Specialist deals an awful lot of damage with shotguns, which are less expensive.
Carry weight increased to allow you to sport all that sweet firepower.
Grenade number is also increased.
Increased mag capacity.
PS : To level up the Support Specialist, you must deal damages with shotguns, and wield/unwield doors.

» Spawn :
Shotgun (lvl 5)
Hunting Shotgun (lvl 6)

» Best weapons :
HSG-1 rifle : can penetrate many specimens when they are queued, fast reloading speed and a lot of mags (use against smaller monsters).
AA12 : Very powerful, full auto, fast reloading, but small mags. Use it mainly against large zeds (scrakes, fleshpounds...)
Hunting Shotgun : Use it instead of the HSG-1 rifle upon preference. The secondary fire annihilates absolutely anything in front of you, but discharge the gun in one shot. It may not kill the biggest specimens in one hit though, but they will feel it. You should use it against tightly packed zeds or big ones. Don't forget that nades deal also a lot of damage on fleshpounds.

» Use :
The Sharpshooter can easily eliminate every type of zeds, but it is mainly aimed at dealing with large zeds (scrake, fleshpound, patriarch). It can be hard to deal with large groups of zeds though, since the SS requires headshots, so it may be interesting to stay behind or with your teammates.

» Perks :
SS weapons are less expensive (except for the crossbow).
Less recoil (easier to land headshots).
Faster reloading rate and increased mag capacity.
PS : To level up the Sharpshooter, you must aim for the head.

» Spawn :
Lever rifle (lvl 5)
Crossbow (lvl 6)

» Best weapons :
Handcannon : Very effective against smaller mobs, high mag capacity
MK23 : Use it against smaller monsters if you've run out of handcannon ammo.
Crossbow : Very powerful when aiming for the head, use it against larger mobs (sirens, scrakes, fleshpounds, Husk).
M99 AMR : Very, very expensive, a single bullet will cost you £250 at level 6, it'll take £5000 to fully load the gun, without taking the gun's price into account (add another £1000). So use is scarcely if you have a lot of money, in priority against the Patriarch (use it in waves only on lower difficulties, and versus lined up zeds or very large ones).

» Use :
Aimed at getting rid of packs because he's loaded with fast fully automated guns. This class isn't very powerful against larger zeds, so use this class when faced with a lot of smaller specimens.

» Perks :
Can see zeds health bar and invisible Stalker !
Every level increases the distance at which you can see Stalkers and health bars.
Faster reloading and increased mag capacity.
PS : To level up the Commando, you must deal damage with assault rifles, and kill Stalkers.

» Spawn :
Bullpup (lvl 5)
AK-47 (lvl 6)

» Best weapons :
M4 carbine : a lot of ammo, but they go fast. Precise (use it again small monsters).
FNFAL ACOG : Very precise, powerful, but low capacity mag. Use it against larger foes or hordes.
SCAR MK17 : Use it instead of M4 upon preference, pretty much the same as the FAL except less powerful and with a less fun noise.

» Use :
The Berserker can kill anything (large zeds, big hordes, etc.). To kill a fleshpound, you must go backward to kite him and hit him every 3s so he doesn't go... berserk on your ass.

» Perks :
Upon leveling, running speed increases. Resilient against most damages.
5 % faster than the Field Medic.
Melee weapons are less expensive, way more powerful and faster.
PS : To level up the Berserker, you must deal melee damage.

» Spawn :
Machete (lvl 5)
Axe and armor (lvl 6). The armor doesn't spawn with you on Suicide or HoE difficulty.

» Best weapons :
Katana : Fastest weapon, but not the most powerful. Useful to kill smaller creeps. You can use it against larger zeds, but it'll take longer to bring them down.
Axe : A little bit slower, but deals more damage than the katana. You can use it against big zeds.
Dwarfs !? Axe : Very slow, but extremely powerful, can send smaller specimens flying, and can stun some largers with secondary fire. Well used, you become virtually untouchable.
Buzzsaw bow: Very expensive, ammunition are too, can stun large zeds and kill them very rapidly (use it instead of Dwarfs !? Axe upon preference).
Claymore: Slow, but über cool. Useful to kill large ones. Use it instead of Dwarfs !? Axe upon preference.

» Use :
The Firebug can burn through big packs quickly with the flamethrower, and kill large ones with the flare revolvers (shoot scrake or fleshpound once and let them burn. Be careful not to stay in the line of sight of the fleshpound while he burns).

» Perks :
Fire grenade (lvl 3).
Fire resilient (Husk).
Can't kill himself with his own grenade.
More ammo.
PS : To level up the Firebug, you must deal fire damage.

» Spawn :
Flamethrower (lvl 5)
Flamethrower + armor (lvl 6)

» Best weapons :
Flamethrower : best way to burn through packs and hordes of monsters quickly and efficiently, and allow your teammates to save a lot of ammo.
Dual flare revolvers : Useful against Husks (which do not fear the flamethrower as they are impervius to fire) and Scrakes, which will have a hard time hitting you if you pepper them. Can also burn the fleshpound before he gets to your team so he'll be easier to deal with.
Husk Fireball Launcher : Can stun some monsters like the scrake, and deals a lot of fire damage against the Patriarch (as it is impossible to carry at the same time the flamethrower, dual flare guns and the Husk launcher, use the flamethrower and flare guns in waves, and buy the husk launcher for the finale against the Patriarch).

» Use :
Best used against groups or even larger specimens (scrakes aren't weak to explosives, so it's better to let a sharpshooter deal with them if you have one in your team).

» Perks :
Can carry more pipebombs.
Demolition weapons less expensive and more powerful.
More rocket, ammo.
More resilient against explosive damages.
Can carry more grenades.
PS : To level up Demolition, you must deal explosive damage (grenades included)

» Spawn :
2 pipe bombs (lvl 5)
Grenade launcher m79 and 2 pipe bombs (lvl 6)

» Best weapons :
Grenade launcher M79 : Not very expensive, effective against packs, ammo is cheap.
Grenade Launcher M32 : Use it against very large groups or large ennemies like scrake/fleshpound (reloading takes time, use scarcely and plan when you have to reload).
L.A.W : Use this gun against the Patriarch, or on the last wave (10) if your team can cover you efficiently. It deals a lot of damage against fleshpounds, groups, and can stun scrakes.
Pipe bomb : Not so expensive for this class, you can land them on the large zeds paths, or behind a wielded door, or even against the Patriarch.