If you just going to start playing Goblin, follow the method below:
This is the current strategy until we see what happens in future updates.
My recommendation is not to open any tunnels until you have enough coins to open the 7th one.
As soon as you enter the game, complete all the tasks and don't forget to collect the daily rewards.
Now, go to the Expedition section. As long as your coins are below one million, invest all of them in the 6-hour option.
If you have more than one million but less than two million, invest one million in the 6-hour option and the rest in the 12-hour option.
If you have more than two million but less than three million, invest one million in the 6-hour option, one million in the 12-hour option, and the rest in the 24-hour option.
If you have more than three million but less than four million, invest one million in the 6-hour option, one million in the 12-hour option, one million in the 24-hour option, and the rest as you see fit.
If you have more than four million, invest one million in each of the 6-hour and higher options, and use the excess to upgrade tunnels. Continue this way.
This is the current strategy until we see what happens in future updates.