Ok, I was able to make 5 legendaries in 1 day and make 500k gold with this tactic
Im going to assume that you can complete some of the missions im saying. If not, tell me, ill give you some other missions.
So, farm mission six of the overlords den on hard mode untill you have 300k-500k gold. No need for refills, because usually you will level up before you run out of fuel, thus giving you a refill
Then, once you have that gold, I usually farm ramboy all the way through my fuel twice a day.
Once you do that, you should have enough materials to make at least 1 or 2 legendaries, and 3 if you get some epics in silver boxes
Also, make sure that you myth items that you have researched on and that you know will be worth for your final mech build.
Hope this helps