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.
On making mechs, focus on your main/1st mech first, DON'T go around making 2nd and 3rd mechs while your first one still looks like a mess.
Do not use epic and higher rarity to upgrade something, regardless of how desperate you are. You'll regret it if you do, many people did, and I'm talking from experience.
There is a slight boost when upgrading an item from the same category. Example: upgrading windigo with broken devourer gives a 1.1× boost. Upgrading windigo with nightmare gives a 1.2× boost.
Focus on one element for one mech, making a hybrid (e.x. physical and electric on one mech) will never work well for you, not even if the items are maxed, since you don't have a single element to keep on using and dealing more damage.
If you have any questions regarding epic and higher items feel free to private message me or ask it here.
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.