Basically it boils down to damaging zeds with perk weapons (or any weapon if you are playing as survivalist)
The secondary xp objectives only count for tiny amounts of xp, like killing a crawler as gunslinger yields 10xp
(on hard or suicidal, can't remember), with a headshot kill being a measly 11xp
This being said, doesn't mean you shouldn't weld doors/heal/go for headshots, but don't make it your objective.
The best way to get XP is to have fun with your friends, preferably in long games. Difficulty I would recommend is hard, suicidal if you're feeling confident (the difficulty difference from hard->suicidal->HOE is very high, while the xp gain difference is very little)
I'm going to break this down into parts, for each of his abilities.
When Hans uses his gas grenades, they can be avoided by simply running away from them before they explode - not too sure why your having too much trouble with this.
Sometimes he emits gas from himself (farts as my party likes to call it) which you can't really avoid if he has just sprinted up to you. Best option from there is to get people to heal you to counter the effect. Also make sure to not stand in the gas he creates, as this deals extra damage if you do.
When he throws out his smoke grenades (triggered by reaching certain amount of health)let him heal. He has a huge damage reduction buff before he heals. You're just wasting bullets if you shoot at him before he can heal and lose the buff he gets.
When someone does get grabbed, heal them! Don't shoot him while he is healing, as he always heals to full health.
When he is in his critically damaged phase (shooting at you with his guns), make sure to use elevation and corners to your advantage. Do not run down corridors or open areas, as he can just pelt lead at you while you do so. It's all about breaking his line of sight (kiting him effectively) and unloading on him when the opportunity presents itself.
You can tell how injured he is by the colour he appears as, going from green to red with sparks. He can be killed in his form with of red with sparks only, from what I've seen so far.
Finally, don't let him melee you. It hurts. A lot. Try to use elevation and corners, once again. However, don't lead him off away from your party.
I won't deny that this boss is hard (after all, it is a boss). He is the right level of difficulty though, and doesn't need too much tweaking in my opinion.
Hope this helps!
Best Answer
There is an indicator in KF2 that tells you in the Trader how much weight you're carrying. It's in the top-middle of the screen. In the example shown here, taken from my own version as it is, I have a weight of 4 out of a total of 15.
There are currently no indicators when actually outside of the Trader other than being unable to pick up weapons.