Is diplomacy necessary?

That's complete nonsense. To have even the weakest of the mentioned creatures join you, you need a large army. And by the end of the first month, you won't have that. Besides, even if you had all of that, any experienced player would easily defeat you. At most, 1-2 monsters from your entire army will move per turn. Moreover, when I played with one person on an L-map, by the end of the second month, there were no monsters left on the map; we were destroying them all to gain levels and upgrade commanders and creatures. Furthermore, I can't understand where you're getting these numbers from. I'm eliminating monsters of level 7-8 in Haven by the second week. You're either playing poorly, playing on an easy difficulty, or playing on some custom-made maps. Overall, you're providing some unclear and inaccurate reasons. I've said everything I wanted to say.