It would be interesting to get into the topic of pay for various types of powerful individuals in the fantasy military.
You have various heroes that are essentially just soldiers but better in every way and will just not die. You have various flavors of wizards who want large sums of pay for their skill and taking them away from their research. Then you have various forms of intelligent monsters that you have either hired as mercenaries or incorporated into your empire.
Really depends. How common are superpowerful people in the setting? How replaceable are they? How long (and at what expense) does it take to train up new ones? What are the values of their cultures?
If the high powerlevel warriors can wipe out huge swathes of armies, then war could become formalized and overall less bloody. Rather than waste thousands of soldiers in battles, you could just have two powerful soldiers duel it out, with the loser surrendering (because why bother sending thousands of men to their deaths?) just like that. If one warrior acts dishonorably and begins massacring the forces/people of the side that surrendered to him, the warriors of other countries may agree to team up to take him down. These high powerlevel warriors could attain high status in their countries as heroes or as nobles or kings. If a kingdom tries to invade another kingdom, their first and highest priority will be dealing with the other kingdom's powerful warriors, either by challenging them to duels, or by assassinating them. The consequences for losing the duel or botching the assassination would be very high, so kingdoms would rarely go to war with each other. There might be a lot of proxy wars with smaller nations.
If highpowerlevel warriors are rare, inevitably almost all of them are going to wind up aligned with one nation or another. Why bother with the insecurity of mercenary work for low level pay when you can settle down in a kingdom and enjoy security and adoration and wealth?
As for monsters, monster ecology is usually neglected in fantasy fiction. If monsters can reproduce very quickly and/or in large numbers (or maybe you can use growth hormones or alchemic concoctions to make them grow faster), then they would inevitably be tamed for use by militaries as expendable soldiers (like how robot armies are used in sci fi). So basically monster squads commanded by beastmasters. Could be very useful for small countries that are lacking in manpower compared to larger empires that have large populations to draw troops from.
Another question is occupation forces. Highpowerlevel warriors might be able to win on the battlefield by themselves, but one guy can't occupy an entire countryside, or even police a single a city. He can only be in one place at one time. You still need normal footsoldiers on the ground to compel people into paying their taxes, checking incoming cargo, arresting dissidents, etc. Similarly, if a country had armies of monsters, the monsters might only be useful on the battlefield. They would have to be extremely intelligent (and obedient) to be able to serve as occupational forces that can police cities and such.
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