Thanks for the comment. This is my mistake, I forgot to indicate that there are several points on the map from which monsters come. These at these points there is always a group of monsters to guard. Withdraw money from them until you are attacked. I thought it would be obvious, but it seems not, since there are comments, a lot of time between attacks is given SPECIFICALLY so that you can finish off the remnants of enemies and you have time left to grind money.
So everything is OK with the balance :)