Let’s come up with a monster.

I suggest the Demiurge unit – there's only one on the map, it can only be hired in one dwelling, and after purchasing the Demiurge, the dwelling disappears. It's like a unit-god. Energy – 120,000 XP, Attack Power – from 12,700 to 38,000, Mana – infinite, like a phoenix, resurrection 2 times. Immune to Light and Darkness magic. To hire, you need 500,000 gold and 500 of each of all resources. The Tear of Ashi looks like a Titan. After the Demiurge dies, it appears in a dwelling, which also appears. It's infinite.