This is a short guide on how to get Hammer weapon kills the most efficient way possible (at least as far as I know). Getting to level 10 with most other weapons is a walk in the park since ammo is plentiful and found almost everywhere, however that's not the case with Hammer weapons

Here's what you'll need

• Pep Signal Hammer Rage
  
  
  Make sure to equip it on yourself as well as your AI Buddy (by pressing E on any selected skill) since it'll give you a small ammo boost from time to time, additionally to your own. The skill refills the reserves of all your Hammer weapons by 10% each time you activate it so it beats farming for ammo in Lab X or elsewhere
  
  
• As many Pep Skill cooldown reduction skills as you can afford
  
  These will (obviously) help you activate your Pep Signal more often, granting you more uptime with your weapons
  
  
• Damage upgrades to all Hammer weapons you plan to use at the time
  They may be small upgrades but they make a difference nonetheless since almost every Hammer weapon is an ammo hog

Begin by stocking up your Hammer ammo inside the Catacombs, it's a safe area and reaching full ammo reserves only takes a couple of minutes

Next up, load the first mission at the mission select terminal (the difficulty will be set to normal by default which isn't a big deal but can be changed to easy at your leisure)

Then, get to killing some nazis. I recommend going through the first two areas in stealth first and returning to them afterward since they offer a pretty substantial experience bonus in addition to many low armor enemies. Additionally I recommend leaving the level by holding the use key after finishing the area before the boss arena as it adds a relatively large amount of time to your run for a pretty low payout

Finishing weapon level 10 on all Hammer weapons took me about 60 runs give or take. It's definitely tedious but also the most efficient way