Please offer some advice.

But what if you need all the units? Or do you only have enough money for unnecessary, weak units?

The 1st and 2nd levels aren't needed) Because it doesn't pose much of a threat, but it costs almost as much as the strongest units.

If you only have enough for unnecessary units, then wait until you have enough for the necessary ones and loot the map. If you don't have enough money for troops at all, then use heroes with the "hit and run" tactic (attack, reduce their numbers, run away, and repeat).