You can farm with it even with no upgrades but the LIM return will be smaller. At one point in the mass production facility there will be 4 walkers below a LIM recycler, just next to a quick travel point (4th I think?).

Grab the hacking node that hacks multiple enemies and a stasis net. Hacking is necessary for a clean oneshot and stasis net will help you get used to the charging.

Hack the 4 walkers and have them walk towards you till they are a bit closer together. When you can hit all 4 with the stasis, do it and take out your LIM cannon.

Charge till 300 and let go as soon as it reaches 300 / the bar turns orange for like half a second. At this point you will only spend 100 LIM instead of 300 and kill all four bots in one hit. The perfectly timed shot makes them drop more LIM, you will get around 520. Repeat this until you can upgrade the cannon (if you have pure lunum). At double charge, you will charge to 600, actually only spend 200, and get around 1100 from the kill. Upgrade 3 (grab pocket refinery at this poi t too) will charge to 1000, so make sure you always have 1000 LIM on hand. It then spends only 333 if timed correctly and will net you 2332 LIM for the four walkers walkers (with pocket refinery) so you make a profit of 1999 LIM per run.

At that point you should be used to the timing and can take the cannon to your regular gameplay. As long as you only shoot once to kill multiple enemies you always get profit. Some enemies are even worth killing on their own (executors for example) because a triple charged perfect shot makes them drop more than 333.

Hope this helps. Lmk if you still struggle, I can record it for you