It is definitely difficult to stay competitive with the top players or in a top guild if you spend nothing at all (though not impossible, I’ve known a few who did it.). That said, you don’t need to spend like a whale to be competitive as long as you are smart about spending your resources. You won’t top the leaderboards for highest total power, but you can keep the critical 20-ish heroes up to snuff by doing the monthly and perhaps a guild gift now and then coupled with good resource management. Once you get to a place where you are able to finish the progress rewards for most contests (and therefore get the double drop), and are able to maintain relatively high in at least Platinum I for FP and Coli (for the diamond rewards), it’s honestly not difficult to maintain that level. Getting there either requires money or hoarding for long chunks of time to get and then make optimal use of double drops.
1) Save stamina for contests where it gives points and where the progress rewards are good.
2) Save diamonds for contests as above.
3) Spend diamonds for stamina only when you have enough to get to the 150 diamond ones that give you double drops if you have a high enough VIP level for that.
4) Buy stamina with guild tokens and bazaar tokens when you can (and stamina cost resets if you don’t have enough)
5) Focus your rune resources on your top 15 and ignore the rest until you have them in good shape.
6) Focus all of your resources on a top 15-20 heroes. Pay attention to what is in challenger FP and Coli.
Yes, it did take well over a year for me to get to where I can easily maintain instead of just barely keeping up. Cyan is a huge step in the right direction for this though, the grind is much more reasonable. O5 to Cyan are pretty much hell if you are not a big spender.