From time to time, your progress will stall for want of a particular resource (if it didn’t, there’d be no reason to spend money in the game 
). Regardless of whether you actually spend money, you are also spending your time playing the game, so it’s useful to think of the best return on your investment.
This will depend on why you are playing the game - e.g. if you want to progress across the different game elements as quickly as possible, it will generally make sense to invest in 1 gun, but if you enjoy the experience of using different weapons then you will potentially forego some progress in order to invest in multiple guns.
Assuming you want to progress across the elements (i.e. be able to complete campaign worlds to unlock new ones, to go further in the tower tournament (once you complete World 5), to complete higher difficulty levels of daily events, to go further in castles (once you get keys from Hard events), etc), the most important resource in the game (i.e. the one you should optimise your game-play to earn and be most careful in your spending) is Hero Medals.
The reasons for this are:
- You need a lot of them:
- To get a hero to Level 50 (a reasonable target if you play a lot, and it’s also the level at which you unlock the 2nd all-hero bonuses) will take 180 hero medals.
- You will ideally need at least 5 heroes at that level (1 each for Poison/Ice/Fire/Magic events and castles and 1 Farmer). BTW The reason to not have a strong Metal hero is that all the accessible Metal heroes are weak in attack, so it’s better to use the strongest of your other heroes
- If you want to get All-Hero-Bonuses from heroes you don’t play with, you will need 30 Medals for the Level 20 bonus and a further 150 for the Level 50 bonus
- It’s hard to farm Hero medals:
- You are guaranteed some for completing daily events (1 for Medium, 1 for Hard, 2 for nightmare; but you need to be pretty strong to complete nightmare consistently) and additionally there’s a chance of random drops - but you are limited to 3 attempts a day (6 if you spend gems on the recharge, which IMO is the best way to spend gems)
- They drop in castles (but at a lower rate than in daily events) but now that castle runs cost energy it’s harder to farm them. And although loot drops have increased, it doesn’t seem like Hero medals drops have improved
- You can’t recover Hero Medals once used - so if you make a poor Promotion choice, you are stuck with it.
- While Rings are also scarce and 4- and 5-star skills are very expensive, generally when you grind enough shards to get a hero to 4/5 stars you will also grind rings (unless you are getting shards by spending a lot of money on summon gates). Also, you can reset rings, so a poor choice will not lose you rings (just 100 gems for the reset and the value of the coins used)
- Shards are also a limiting resource, but unlike Hero Medals and Rings, they can only be used on the hero you earned them for - so once you get enough to star up a hero there’s no reason to defer it. And Castles drop more Shards than Hero medals.