I've had a series of rough, rough games vs Hans, with people who don't know how to handle him, and it's frustrating. So here's a Public Service Announcement. A lesson in how to neutralise an antagonist Hans.
Don't cower in cover
If you're stationary, you make it incredibly easy for him to hit you with his weapon. The splash radius of his bubble gun and rockets are very small, and the projectiles move slowly. If you're sprinting and sliding, it's very hard for him to hit you.
Stay within Line of Sight of your allies
If you can shoot him down before he reaches his target, you can finish him together.
Grapple if you are brawling with an ally
This is a general antagonist rule, but if there are at least two of you, and neither of you are one punch from death: grapple. The only way he can escape is by dodge-rocketing, grappling him will stop this. Which leads me to the next tip.
Stay in pairs
If Hans does manage to down someone, make sure you're close enough to interrupt him from K.O.ing your ally. As he nears your ally, start punching him, don't go for the grapple. He will be desperately trying to get the immunity window from killing your friend. If you delay him for a few seconds, your ally will come out of the downed state and assist you.
Hunt him down as a team
Don't touch the objectives until the antagonist is dead. You'll have a 20 second window to continue with the objective once he's out of the picture. Always have your eye on his respawn timer, and once it's up, locate and destroy him.
Pick the right raiders
Some good counter-picks to Hans are Alicia, Ginebra and Hans. They are very good at hunting him down and tripping him up. Don't pick Shae or Harec unless you're extremely confident, Hans will tear them up otherwise. If your team looks strong or you can trust that they are reasonably experienced, then take a sniper, but I wouldn't advise taking both of those characters together. As a general rule, it is best to try and have a balanced team. Konstantin is a good secondary, supportive pick because his bubble will save a lot of lives. He's an excellent babysitter.
A lot of these tips are good against all of the antagonists, but I had to make this thread specifically because I see new and even experienced players falling into the trap of hiding in cover against an aggressive Hans all the time. Be aggressive in turn and he's helpless.
