Hans received significant changes to his mechanics, and all of the weapon damage resistances have been altered as of the Sharpshooter update. The information in this guide is now wildly innacurate, and I don't have the time to update it right now.

There's a google sheet out there with the latest numbers and mechanics on if you do a bit of work on the google machine.

Hans had a few very significant changes made to him in the Incinerate & Detonate update that aren't covered in this guide, as it was written quite some time ago. Here's the essentials on the changes:

The first massive change: Hans, like all the other zeds, now has different damage multipliers based on the materials from which his body parts are made.

The most notable effect of this is that shooting his arms, legs or exoskeleton will now deal only half damage with all weapons. Aim for his fleshy parts (or his power cores, see the next point) unless you like wasting ammo.

The upside to this is that the big glowing power core in the middle of his backpack, and the smaller one over his heart, are now vulnerable zones. Both power cores, along with Hans' head, take 10% more damage from all sources.

The second massive change: Hans now also has global damage multipliers, which stack with the location effects already mentioned. He takes half damage from toxic, explosive, slashing and EMP and 25% more damage from the microwave gun. This makes Ballistic, Piercing and Blunt weapons (i.e. regular guns, the Zweihander stab attack, and the Pulveriser) and the Microwave gun the best choices for arming yourself.

Also Hans now has a visible, transparent red shield around him when he enters hunt mode. This makes his damage mitigation more obvious to players, and also acts as a good indicator for when to let him catch up to you.

Weapon Breakdowns:
All perks
Perk knives and the Katana deal slashing damage, and as such should be avoided. The 9mm deals ballistic damage though, so at least its mediocre damage isn't reduced even further.
Berserker

• Crovel: Light attack is slashing, do not use. Alt fire is blunt damage, can be a last resort. Doesn't have the parry rating to be able to parry Hans' attacks.
  
• Pulveriser: All attacks deal blunt damage, and the weapon is capable of parrying Hans. The explosive blast from the alt fire won't hurt him much, but the heavy attack also has a significant blunt damage component as well. Actually works out to slightly more damage per hit than the Zweihander heavy attack including the explosion, and about the same without - but the Pulveriser swings faster.
  
• Zweihander: Light and heavy attacks are slashing, so they deal half damage. Bash attack is piercing and deals relatively heavy damage, and the weapon is capable of parrying Hans. Although the heavy attack does twice the damage of the blunt impact of the Pulveriser's heavy hit Hans has 50% resistance to slashing and the slower attack speed is a hinderance. Poke him inna face.
  
• Eviscerator: Now capable of parrying, but deals exclusively slashing damage. Not really viable, especially as it deal continous rather than burst damage (making hit-and-run far less effective).
  
• EMP grenades do 50% damage, but you're not lobbing them for the damage anyway.

Commando
All your guns do ballistic damage, so you're fine. You're a Commando, same approach to every zed - shoot him in the face till he falls down (or the backpack power core if he's turned the other way). Your grenades won't hurt Hans much, as they're 100% explosive.
Demolitions
All your weapons do mostly explosive damage, with additional small blunt impacts on direct hits. You're basically completely useless against Hans, as he has 50% resistance to the majority of your damage. Ideally, switch perks. If you want Demo XP from the boss, hold on to your C4 and tag Hans with it before swapping to another gun - but don't fight him with an RPG. You'd be better off swapping it for either of the other 10 weight weapons - even off-perk. Your grenades won't help much either, as they're 100% explosive.
Field Medic
Again, all your guns do ballistic damage - but as you lack a perk damage bonus, you might want to pick up something beefier alongside your HM04 AR. A Pulveriser for a last-ditch melee attack is probably your best bet. Toss your weapons and grab a Microwave gun/AA12 off the floor if everyone else is dead. Your grenade's toxin won't really scratch Hans thanks to its low base damage and his 50% resistance to toxin, but their healing is invaluable.
Firebug
This is an ideal matchup for you - Hans takes 25% more damage from the Microwave gun, on top of your perk bonus, and has no special resistances to fire damage either. Hit his torso/head/core with MWG bursts 'til you're out of ammo, then pick up one of the other two+ MWGs you bought (you did remember to do that, right?) and empty that one as well. Continue until Hans explodes violently, or until everyone else dies and he rips your face off. Remeber that your damage falls off over range, so it may be advantageous to swap out extra splash damage for extra range during the boss wave. Your Molotovs would initially seem to be great, as Hans has no fire resistance - but remember, the fire will be hitting his legs and only dealing half damage as a result. Toss them anyway, but don't expect them to save you.
Support
As with the Commando, all your guns deal ballistic damage. Just aim your AA-12 at his face/backpack core. Your grenades also have a piercing component, and deal extra damage thanks to perk bonuses - don't be shy about lobbing them in.

So, Hans Volter. A lot of people are having trouble with him, and after some discussion and help from across the internet, I've hunted down some information that might help you deal with the ageing Nazi psychopath.

Hans has two "modes", which I'll be calling "Relaxed" and "Hunt", and 4 phases. He begins the encounter in phase 1 at 100% health, and in "Relaxed" mode. In Relaxed mode, Hans pursues the group and uses all of the attacks available to him in an attempt to kill them. In Hunt mode, Hans sprints after one team mamber and attempts to drain their health to heal himself.

The phase number dictates which special attacks Hans has available to him, with increasing phase numbers unlocking progressively more dangerous abilities as well as augmenting his existing ones. Higher phase numbers also grant Hans increased movement speed and damage.

In phase 1, Hans will walk around relatively slowly and alternate between basic melee attacks, firing his dual StG44's, and throwing pairs of gas grenades. In phase 2 he gains the ability to throw frag grenades, and gains extra grenades in his gas barrage. In phase 3, both grenade attacks gain significantly more bombs and Hans will begin to use his rocket pack to jump around and pursue players more aggressively. In phase 4, Hans goes berzerk and will relentlessly sprint after the players, generally sticking to melee attacks but still employing his grenades occasionally.

• When he drops below 35% health and is not in phase 4, he goes into "Hunt mode" and increments the phase counter by one. The phase counter determines his damage bonus, what attacks he has available, and what colour he glows. If he's in phase 4, he can't enter "Hunt mode" any more.
  
• In "Hunt mode", Hans gains 85% damage reduction against all attacks. During this time, he constantly talks about draining someone and sprints after the team until he successfully grabs one of you. This is the point where most teams waste all their ammo, doing dramatically reduced damage as they kite him when he's in "Hunt mode".
  
• When he catches someone, he incapacitates them for a few seconds and drains their health, and drops a spread of smoke bombs. The whole team should be healing the victim at this point.
  
• When he releases his victim, his health resets to 100% and he returns to "Relaxed mode", returning to fighting normally and losing his damage reduction. He also plays a brief animation that keeps him still for a few seconds, which should be used to allow the victim to run the hell away so they don't immediately die when he starts attacking again (and for everyone else to shoot the crap out of him).

So, there are two basic ways to kill Hans:
A) Do not let him catch his Hunt target, and burn through the last 35% of his HP despite the 85% damage reduction
B) Allow him to catch his Hunt target so he heals and transitions to the next phase, and repeat the process until he enters phase 4 and goes berzerk - at which point he can no longer heal.

Both phases require approximately the same amount of damage. Both need to begin by dealing 65% damage to Hans, then:
Path A) has to chip off the remaining 35%HP through 85% damage reduction, which is approximately 233% effective health - for a total of 298% of Hans' base HP in damage required.
Path B) has to deal 65% damage to Hans twice more, then a final 100% to kill him once he goes berzerk, for a total of 295% of Hans' base HP.

Path A requires absolutely perfect kiting - if Hans catches his target, he will restore himself to full health and you will have to fall back to Path B. However, you won't have to deal with Hans' more devastating upgraded grenade barrages as he never progresses beyond phase 1.

Path B is easier to co-ordinate, but requires you to stop shooting and deliberately allow Hans to catch you when he enters Hunt mode - any damage dealt to him after triggering Hunt mode is wasted, as you won't be killing him before he heals. You'll also have to survive the massive barrages of grenades he throws out in later phases, and his final sprinting charge in phase 4. Berzerkers can, and should, block Hans' melee attacks to allow their teammates time to shoot him. This makes the Eviscerator a poor choice for this boss fight, as it is incapable of blocking. The Eviscerator can now block as of Incinerate & Detonate, but is still a poor choice for the reasons stated in the new section on that update above.

TL;DR : If you're playing with pubbies, let Hans heal as soon as he tries to do so and heal the victim to keep them alive. Once he heals three times, unload on him with everything you have to bring him down after he goes berzerk. Kiting Hans so he can't heal is possible, but doesn't save you any effort as he only takes 15% damage from your attacks while trying to drain someone. Shoot him in the head, or the glowing cores on his backpack or chest.

The fine people in this reddit thread, in particular MOOSiEMAyNE and Kuebic for correcting some of the stuff I got wrong initially.

And, of course, Tripwire for making this game.