Mythos Slayer is an idle boss-rush game where you mash to summon allies and grind down bosses from the Cthulhu Mythos. The basic controls and where the upgrade tabs live are covered on the game page's how-to-play section, so here I'll go a step deeper into where to put your effort to grow fastest.
Where to spend your first upgrades
There are plenty of things to spend coins on, but when in doubt, early on you can't go wrong starting with weapon upgrades. Once each hit lands harder, the boss in front of you melts at a good tempo and your coin flow suddenly picks up. Click power is the foundation, so if it's thin, nothing else really takes off.
Once your damage has some room to spare, move on to coin gain and speed. Coin gain is the fuel that turns every other upgrade, and speed raises your raw number of actions. Thicken them up in the order weapon then coin gain then speed, and your earnings naturally accelerate.
Make idle time the main way you earn
In this game, the time you're away from your desk is prime earning time. Your commoners gather coins, your soldiers keep hitting the boss, and since bosses revive after they fall, coins pile up just from leaving it running. Before bed or when you step out for a bit, beef up your automatic earners. That alone makes the next morning's coin total wildly bigger.
That said, it isn't built so that progress stalls the moment your hands stop. There's time for hard mashing and time for just leaving it be, and either way you keep moving forward, so the trick is fitting it around your day.
When you stall, prestige is the answer
When progress starts to feel heavy, that's your cue to prestige. Prestiging raises your coin multiplier; your progress resets, but the next loop becomes dramatically lighter in return. Rather than hoarding it because it feels wasteful, going for the multiplier is faster in the end.
As you repeat prestige, upgrade, prestige again, your soldiers' damage keeps gaining digits, until at last you're mowing bosses down with numbers almost too big to read. That moment of "the numbers breaking down," so characteristic of idle games, is the tastiest part of this one.
The right way to grow is different for everyone, so start by getting a feel for Mythos Slayer and finding your own approach. If you're curious why I designed it this way, I've written about it in the dev diary.