There's a very easy way of doing the following things:
-Level Up Characters
-Level Up Your Account
-Earn Lots of Gold
Do the following steps:
-Custom Game
-Private Room
-Crew Battle
-Set to 99 Lives
-Set Time to 25Mins
Then, select a bot or use a secondary account and go to a Flat Stage like Small Brawlhaven. As soon as the battle starts, pause the game, exit the match, rejoin the match by pressing A or X (Or C if you're using a keyboard) and now your character will be fighting by itself as long as you don't press any buttons (if you're using a secondary account, you need to do this process with it)
And now that the characters will be fighting by themselves for 25 mins, you're free to do whatever you want while you wait, such as watching Youtube Videos, watching a TV series or movie, reading comics, drawing or anything else you like to do with your life. After the match ends, your characters will earn over 900XP and over 200 Gold, as well as earning XP for you Account, if you continue this process, your characters will eventually level up enough to earn you achievements, your account will level up enough for the achievements and you gain enough gold to get their respective achievements, just repeat this process until you do get every achievement you need, it will take time and a lot of patience but you'll get it with time
If you prefer a video, here it is: