What should I do when Hans grabs a teammate?

Do not shoot him; heal the victim. He takes heavily reduced damage while draining, and extra damage only makes him drain more.

What changes after Hans has healed three times?

He cannot heal again. Throw grenades and deal as much damage as you can as fast as possible while keeping teammates healed.

Tactics

Hans Volter Quick Tips: Community Rules for KF2

Several short early-access guides and their comment threads boil the Hans Volter fight down to the same few rules. This page merges them. For the mechanics behind them see the detailed hunt mode breakdown.

Core rules

Stay with the team. If Hans comes after a lone player there is no help. Do not abandon the group.
Run and keep your distance. Hans is fast, so keep moving; never stop.
Dodge bombs. Avoid his gas and explosive grenades at all costs; they do mass damage. White smoke bombs mean he is healing.
Save ammo. Use melee on easy targets and keep ammo for the fight.
Medics do your job. Heal teammates with any medic weapon (alt fire, by default the middle mouse button, fires a homing dart).

When he drains someone

When Hans grabs a player and sucks out their health, do not shoot him. He takes much reduced damage in this phase, so shooting is a waste of ammo, and in some reports extra damage just makes him drain more. Heal the victim with a medic gun, or even run up with a syringe. Use the time to clear nearby zeds, heal teammates and reload.

Keep the healthiest player as the feeding target if you can, and keep healing them: Hans resists damage until he has healed up.

After the third heal

Hans can drain health three times. After he comes out of the third heal he cannot do it again. This is the moment to spam grenades and dump damage as fast as possible; with continued healing he should go down.

If the medic is the last one alive

A medic left alone with Hans after the third heal can still win:

lead him around and shoot only when it is safe;
use doors: closing one in his face slows him and blocks grenades and bullets, and stops grenades from bouncing back to you;
if you are hurt or need to reload, wait for him to throw grenades at you, otherwise heal and leave.

Perk one-liners

Support: a popular joke guide boils it to "equip Support Specialist, buy the AA-12". It is a good start, but keep reading the fight.
Berserker/other perks: see the per-perk lines in the detailed guide.

FAQ

What should I do when Hans grabs a teammate?: Do not shoot him; heal the victim. He takes heavily reduced damage while draining, and extra damage only makes him drain more.
What changes after Hans has healed three times?: He cannot heal again. Throw grenades and deal as much damage as you can as fast as possible while keeping teammates healed.