This is achievable with the "Leech Hunter" "cheats".
1. Finish the game on normal (or hard if you feeling like it but not adviced) with rank S. Save the game in slot 20 or a slot you would never ever touch. During this run make sure you make a "Custom Pistol" for either Rebecca or Billy. This is usefull if you want the "I Could Shoot, You Know" achievement aswell.
2. Go and play "Leech Hunter" and obtain all the 100 leeches. Remeber that you can never drop the leeches so I recommend first killing every enemy and than collect all the leeches. You can also get the "Distracted from the Hunt" achievement by killing all the enemies. Remember that after collecting 90 leeches, 23 enemies have respawned. You need to kill those aswell if you want the "Distracted from the Hunt" achievement. After collecting all 100 leeches, finish "Leech Hunter".
You will now have inf ammo on all weapons, Rocket Launcher, SMG and Magnum Revolver.
3. Load this save file again and click "Once again". Go through it easily without saving and get the "Save Your Prayers" Achievement. During this run make sure you now upgrade the other pistol to custom to get the "I Could Shoot, You Know" if you were going for this.
This is the way I did it and it worked.
Cheers!