Rolling initiative as GM involves rolling 1d20 and adding the Level of the enemy the party is encountering. To diversify combat, you may separate enemies into three categories: Minions, Leaders, and Specialists. Each of these enemies may roll separate initiatives, however to avoid the party getting swarmed by every enemy taking their Turn together, it is smoothest to have some enemies hold their Actions until after some party members have taken their Turns.
Enemy stats are super simple in Arcane Dominion. As the GM, simply use the party's level as your roll modifiers!
Enemy HP pools are [10 x the party Level] (minimum 10), and their defenses are all [10 + that number]!
You can add challenge by boosting enemy stats to the maximum amount for their given level, which can be handy to make a boss feel more challenging.
If you are going to have an enemy fight the party solo, then use 100x the party's level for its HP.
Not every encounter must end when the last enemy is defeated; it is best ensure each player has a chance to take their Turn. If an encounter would end before every player has had their Turn in a Round, you may implement a "Rout". Routs are when the party has overwhelming force and can finish an encounter by having each remaining player who has not yet had their Turn perform an attack roll and you as GM can describe the encounter ending in a flurry of blows defeating the remaining enemies in a montage. Alternatively, if every party member has taken their Turn and left onle a meager enemy presense, have the remaining enemies retreat.
These grunts should be ready to fall in one hit, so don't be afraid to add lots of them. Either set their HP to 1, or have their stats as the minimum values based on the party's Level. While it is easy to take them out, don't underestimate the strength of a horde!
The enemy may have one or more leader character, and taking them out might end a fight early with the remaining minions losing confidence and running away. Be sure to give these enemies some tactics and defensive maneuvers, they should not fall down easy!
Since Leaders will often go first in Initiative, you can have them Command their Minions.
These enemies should have some of their stats maxed out for the party's tier of gameplay. For Adventurers leveled 0-5, that would be ~10. For levels 6-10, ~15. Continue adding +5 for every set of 5 levels beyond level 10. These enemies can therefore pose a real threat, they should hit hard or be hard to hit!
Providing Cover for characters to hide behind can make a battle more dynamic. Adding further details or interactable objects helps players become immersed and creative with their actions. Encourage your players to duck in and out of cover, leap over gaps, push enemies into water!
Add variables to your encounters every once in a while. Most species in A.D. do not have a way to see in the darkness. High winds may affect projectiles. A heat wave may Exhaust those heavy armor, while exposure to cold or wet conditions can make things Slippery.
The characters you as GM play can have the biggest effect on how the players experience your world and story. As the GM, you are the story-teller, and narrating the player's actions as they explore your world is the primary method of exposition the players receive; however, the players will be much more likely to remember a quirky character after a session than the way you described the mountains.
The best thing you can do when plotting out your story characters, is to assign them a core motivation. How does this character live? What do they need to survive or thrive? How do they intend to get it? What is getting in their way? How a character's motivations compare with the party's creates the story for you.
Sometimes, something completely random and fantastical takes the cake. Regardless of your setting, when you need to throw in a character to be an innkeeper, shopkeeper, or quest-giver, consider giving them some random quirk that makes them stick out from the crowd. Especially if you want a character to be remembered, make them easy to remember! Could be an eyepatch, dyed hair, or a cool pet.