Hero Coins can be obtained through various ways in the game.
Achievements
Completing Achievements will grant you with a certain number of Hero Coins (depending on the Achievement). However, these can only be claimed once after completing the Achievement.
In-Game Events
There are some in-game events that will grant users Hero Coins. Certain events happen weekly, such as the "Weekend Party" event. This will grant you 200 Hero Coins every weekend (Friday - Sunday).
Then there are other random in-game events that will grant you Hero Coins for completing tasks. Be on the look out for these events (can be found in the Announcement icon, next to the Gift icon located in the Main UI).
Daily Targets
Completing the missions in Daily Target and gaining "Activeness" will reward you with Hero Coins along with other rewards!
Hero Coins are obtained from Activeness Rewards 10, 30, and 50 for a total of 80 Hero Coins per day (when completed).
Weekly Targets
Similar to Daily Targets, there are Weekly Target missions you can complete to earn some Hero Coins. Completing these two missions will grant you with 80 Hero Coins.
Arena
There are Weekly Quests in Arena that you can complete to obtain 120 Hero Coins. For more information head to VS -> Arena -> Quests.
There are also Season rewards that reward you with Hero Coins depending on your final placement at the end of the Arena Season.
Daily Sign-In
Make sure to Sign-In everyday, as you can also obtain Hero Coins from the Daily Sign-In calendar. Days 7 and 15 will reward you with Hero Coins.
There are still plenty of other ways to obtain Hero Coins through the game, such as progressing through game, leveling up, ranking up in Super Coop, etc. However, the ways highlighted above are the guaranteed ways to obtain Hero Coins daily/weekly/monthly.