In almost all ways possible to play it, PvE combat is one of the lower paying activities in Elite.
There is however one decent grind, stacking massacre missions, which works as follows:
Find a system with good hunting opportunities (HazRES, station or fleet carrier nearby)
Check whether the surrounding systems spawn pirate massacre missions targeted at that hunting system. Wing missions preferred, the more factions offering some the better. The less rivaling target systems nearby the better.
Use INTRA to find systems meeting these and other requirements you may have, like Federal or Imperial minor factions, or Powerplay alignment.
Stack pirate kill missions
Not war / conflict zone missions.
Important: Kills only count for multiple missions if the missions were given by different minor factions, so keep track of kills per faction, but try to take as many missions as you can.
Wing missions are preferred because they can be shared and have much better deadlines.
Choose stations close to the system entry. Mission boards refresh partially every 10 minutes, in sync with full hours.
It is recommended to keep track of the kills per mission issuing faction in a spreadsheet to avoid overstacking for few factions only
Farming is more efficient and fun in a wing, and since wing missions are shareable this also multiplies your credit gain.
Getting allied with the factions in question and having good combat rank yields higher payouts. This tops out at ~ 500k per mission per kill. Assuming you get the perfect stack of 20 missions with no duplicate mission issuers you would get up to 10 Mill per kill.
*Check faction status - Factions in war/lockdown/similar won't give you any or only very few missions!
Notes
Multiple HazRES available to hunt at
Fleet Carrier required in Albardhas to gain quick restock
Missiongiver Systems
Huan Guang: Anthony Landing (Outpost)
Choerini: Beshore Survey (Outpost)
Cuberara: Sabine Camp (Outpost), Antonio de Andrade Platform (Outpost)
HIP 104504: Stein City (Outpost), Baillaud Ring (Large)
Notes
Hunt at Tjindjin 6. 3Rs available at outpost.
Missiongiver Systems
LTT 4961: Greg’s Legacy (Asteroid Large)
Catarihas: Hardwick Hangar (Outpost)
Weleserme: Harrison Settlement (Outpost)
LTT 5013: Margulis Station (Large)
Use INTRA for many filtering options and a live view of which systems have factions in good states.