If you're like me and didn't spend (or have) 11 ability points for recharging Kraft ammo upgrade then:
A good place to boost this is in Siegturm Plaza. It constantly respawns around 15 enemies and ammo boxes for Elektrokraftwerk after you leave through the gate to Riverside and back.
Alternatively you can use both Riverside and Siegturm Plaza to go farm kills back and forth however in my own farming run I found that Riverside just doesn't respawn that many ammo boxes as Siegturm Plaza not to mention Riverside is also a bigger area.
During these runs you will almost always encounter Panzerhund, Zitadelle or Zerstörer (giant robots). Bigger Guns upgrade that allows you carry heavy weapons is very useful here and if you encountered any of them before you should have obtained Uberhammer which acts like a nuke (or BFG from DOOM) and kills those robots with ease, if not upgraded LaserKraftwerk is a good alternative. Regardless killing any of them 3 will always drop you more Uberhammer ammo