On Mhenga, you can farm the Vanae for their spears which have a price of about 1200 each. It's not bad money early on, but I usually don't spend more than 1000 credits early on for anything. The space pirate outfit and the shocker pistol with the decent Joyco shield is an excellent starter pack that lets you handle most situations on Mhenga easily. The only problem with this method is that the merchants suck, so I tend to save my more expensive gear and items until I run into Anno.
Once you hit Tarkus, the gray microbots and spunkshrooms/motherhusks give out pretty good cash, and don't forget there is a free horsecock that you can always find on the "Next to Crashed Ship" tile. Those sell for a nice 15,000 each, and you have a small chance to find more wandering the wastes after the first one.
I don't farm Myrellion because by that point I have access to Uveto, but you could go down and farm the silly mode goo knights for gems, since they have a guaranteed kirkite or satyrite drop if you do a victory dicking.
On Uveto you could farm korgonne for their items, but it is much easier and more consistent to simply save scum the horse racing for 10,000 each time. This is probably the most efficient and risk free farming method in the game (excluding Lash), and with around 300,000 credits you already have all the money you will need for a long while.
If you want serious bucks, you will have to farm Lash. It is honestly not that hard, for any character. The class doesn't matter anymore so long as you have fully upgraded grey goo armor (the defense upgrade probably doesn't make a huge difference, but it is extra defense points so why not), a good weapon for your class, and a salamander coat (purchased from Emmy on Myrellion). It is advised to also use a crystal shard before battle, and to do a dancing lesson with Finn and to drink Aina's tea for an evasion boost and a health boost, respectively. With this, you have 75% thermal resist (his weapon does 28 burning, according to the wiki), 12 defense, how ever much evasion you have from your choice in weapons and Finn's boost, and hopefully a consistent weapon for damage. His moveset is limited to only one attack: a very highly damaging attack, but the bonus from the armor and burn resist should leave you taking only between 3-6 damage per hit (this was tested without the crystal shard), with six hits maximum. At worse, you will take 36 damage on one salvo, which would be on the lower end of damage from one hit without any burning resist. In this fight, burning resist trumps all, and if you find yourself still taking too much damage, consider getting the thermal underwear for another 10% burning resist and maybe some health boosters from Nenne. This fight can be remarkably easy if you prepare right, which may include maxing at least your most relevent combat stat. Something that is very important to remember is that shield resistances and armor resistances are separate, which is why your shield will take full damage while your health bar does not. So, don't bother getting a salamander shield in the hopes of increasing your burn resistance to 100%, because it doesn't work that way.