Depends on the thing. In general, once you've completed 1v1 on normal, you can try out hard mode on the first three chapters (dry lands, etc) as they're easy almost every time. For grinding fortune boxes, you can go to the dry lands boss on normal. (Hard mode or insane if you want to) Note that there is a CHANCE for a fortune box, it's not guaranteed. Overlord's Den 2, or 3, or 6, for grinding coins and XP. The most efficient way is on hard mode, but if you can't do them yet, stay on normal mode. Thats most of the general grinding things you need to know.

Any other tips you have for progressing?

Actually, yesterday I found that if I upgrade a part with food that has the same element and category, it gives a 1.1x and 1.2x upgrade bonus. So, I am upgrading Nightfall with side weapons and Nightmare with torsos, and it really helps me upgrade faster compared to what I used to do before. Regarding Savior Resistance, I will change it ASAP when I find something good to replace it with. I was holding the Eagle because I wasn't sure if it was better than Desolation or not, but with your comment, I am now sure I can change it. Thank you! Now that I think about it, evolving a leg wasn't a good idea. I should have evolved weapons first. Do you think evolving mods to myth are a good idea? After evolving weapons and body parts of course.

In order, you upgrade your headquarters, then your gold mines. Make sure they're at least at the same level as your headquarters. Then once you reach level 15 headquarters you can focus on the item factories, at level 15, you can craft power units, which will be really useful for grinding and upgrading.

Thanks for the reply, since im stuck with the base, what would be the best way to get the most out of it?

Quick question, from where did you hear this? ——— Base deactivation has been disabled a long time ago, to no one's surprise, base is actually better in the long term. While getting silver boxes might be cool, you're only gonna get items up to epic and no more. Not only are the silver boxes gonna be more expensive the more you progress, you will earn less from the boxes since they only provide oh so small amount of item. In short, stay with base, even though it takes a painful amount of time to max it, it will be worth your time if you play your cards correctly.

Base is actually useful, you don't need to disable it. Even if you want to you can't.

I heard deactivating your base is the best way to progress, how would I go about doing that though?