Editor's Note: This article was written before skills & gear were reworked. It is still worth a read, but in order to stay up to date, read my new article after reading this.
https://app.warera.io/article/69dd5e17e28eff0d47b265c7
Last time we talked about early game, now we'll take a look at middle-game. (Fyi, I consider somebody to have reached midgame when they're +20 lvl, or they have +6 lvl 4 companies. I never saw the term used in the community, so this is not a meta term. Just my observations.)
There are a lot of newbie guides but so far I haven't seen any for +lvl 20 players, so I wanted to write one. There might be inaccuracies or mistakes within the guide, if you see any, please point them out.
Anyways, to the guide.
Situation:
Unlike the early-game where you pretty much have to follow the meta in order to have success, you have several options now. With your companies set up and upgraded, you should have a decent daily income with which you can support several ventures.
1. Staying eco:
If you'd like to, you can stay in eco longer to ensure you're better prepared before you switch. Some people like this option better, I personally find it extremely boring, it's totally up to you. If you do end up staying eco, it's pretty straightforward. Just keep creating new companies and upgrading.
2. Switching to fight build:
Your other option is to switch to a fight build. Now, this requires some preparation beforehand, especially if you're on the lower spectrum of lvl 20s. We'll look at this in parts to make it easier to read. At the end, I will give you two examples to show that different builds are viable.
A. Skills:
Fight skills are not as simple as eco skills so it might be confusing at first glance. I'm not much of an expert but this is the build I used and it served me well so far.

From what I understand dodge seems to be the most important skill. Try and get at least 6 lvl dodge. My only regret with this build is not getting enough armor, if I assigned a few more points into armor I think I'd be doing much better. Another regret I have is disabling 3 companies to free up skill points for the build. I switched to fight mode when I was lvl 21 so it was kind of needed, but in the long run it cost me quite a bit and this decision will force me to switch back to eco earlier than I want to. So, if you intend to switch as early as me, I'd advise you to either bite the bullet and accept you won't have as good a fight build or plan your companies accordingly.(i.e., have 6 lvl 6 ones instead of 8 lvl 5 ones)
There are two websites that could help you optimize your build. https://war-era.vercel.app/ can pull your stats from warera and give you an optimized build, though make sure to select the gear you plan to use before clicking optimize because the calculation is based on your gear. I've been told it wasn't totally accurate but it seems like the best one out there.
Another website you could use is https://arcanaera.netlify.app/. This one doesn't have an optimize option(that or I'm part blind) but it calculates damage output so if you have the time, you might wanna mess around and see which build would be the best.
ps: unless the decision is reverted, dodge stat will be nerfed sometime soon so the whole meta is likely to change, when that happens I'll edit this part.
B. Fighting Gear:
You will need about 10 equipment sets(one of each eq type forms a set), to be able to keep fighting for a week. Of course this depends on your activity but generally speaking, 10 should be enough.
You'll need more weapons than eq sets, 15 or so I think. The reason for this is that when you hit and get a dodge, your equipment does not get damaged, but your weapon still loses %1.
The quality of equipments you can use depends on your financial situation.
Generally speaking, if you're over lvl 20 and developed your companies properly, you should be able to fight with mostly blue gear / purple weapon with some purple gear and tanks reserved for important battles.

If you want to fight for a longer duration, you should consider using green/blue sets & blue/purple weapons, switching between them depending on the importance of the battle you're participating in.
Fighting this way won't result in huge damage output and you won't be able to win rounds alone, sometimes you won't even make a difference, however this is consistent damage and over time it adds up. Dealing consistent damage is important because as you grind more damage, your military rank will increase which in turn increases your attack bonus, enabling you to deal more damage.

To summarize:
Blue/Purple Sets: + more damage - sooner switch to eco.
Green/Blue Sets: + more consistent fighting - less damage output in the shorter period
C. Supplies:
If possible, try and buy supplies during peacetime when the prices are low.
As food, you should stick to fish as much as possible since your hunger is limited and should be used as effeciently as possible. For a build which has 8 hunger, you'll need about 100-150 food for a week of active fighting.
As for ammo, heavy ammo is not feasible at this level. I mean you could use some when you're hitting with tank but it really isn't worth it since it'll burn your gold FAST. You can use a combination of medium and low ammo depending on the weapon you use. Medium with purple weapons, low with blue. That seems to be the meta anyways, however I personally have been using medium ammo only and so far haven't run out of supplies, so it's not that expensive. Unless you're planning to keep fighting for more than a few weeks, you should be fine using medium ammo only.

D. Pills:
Consuming a pill increases your base attack by %60 for a duration of 8 hours, then it decreases it by the same amount for 16 hours.
Pills may seem a bit expensive, but they are really worth it. The buff is obviously huge, but the real advantage of using pills is that during debuff you won't be using any supplies/gear, so it's more economic & helps you deal pretty much the same overall damage and save money while doing so.
Though that doesn't mean you should be using them whenever you want. Try and plan ahead before taking pills to maximize the buff time you get.
E. Other Buffs:
There are other buffs you can take advantage of apart from pills. Hitting in battles where your MU/country has an order will help you deal more damage and grind rank easier. Unless you're authorized in your country/MU, you won't be able to set these yourself but if you get the chance, use it.

Two Examples:
I was able to gather 10 blue sets & 3 purple sets with 1 tank and 8 or so purple weapons before switching to fighting mode and so far I've hit with blue gear & purple weapon, using 1.5 sets of purple equipment & 2 tanks for more crucial battles. I also utilized the extra country/mu buffs as much as I could and after five days of fighting, managed to deal about 1.3 million damage.

https://app.warera.io/user/695d8ff7b2e9a2535e1308bc switched to fighting mode at the same time as me. He was ahead 2 levels but he invested more into company skill so that evens it out.
Unlike me, he mostly used green ammo and didn't have access to the sweet %15 country order bonus I had thanks to guyana. Naturally so, as you'll see he did less damage than me. However, since he kept his companies running and used cheaper ammo, he will be able to fight much longer than me and overtake me in damage in the long run pretty easily.

This shows that both approaches are viable and it all depends on your needs. So, even though there is a meta, there is no one-fits-all strategy. Therefore, you should try and adjust the meta according to your situation, wealth and aims.
That's all for this guide. In the next one, we're going to talk about different ways of making money while at fight mode.