Okay, to review magical conflict in Harry Potter, let's start with the known weapons. Wizards use wands. Goblins use swords and spears. Centaurs use bows. Giants use clubs. House elves directly use magic. Dragons breathe fire. Beyond that everyone uses tooth, claw, and scale to fight.
For defense, wizards use enchanted robes and light armor. Goblins use metal armor. Centaurs have light leather armor.
The principles of combat are distance, direction, speed, force, and awareness.
The three casting branches of Harry Potter are charms, transfiguration, and hexes. Charms changes the traits of an object. Transfiguration changes the form of an object. Hexes interact with the target.
Notably, charms and transfiguration both interact with the field of battle rather than directly with the enemy. Therefore the wizard field of battle would be as follows. First, use transfiguration to give yourself an advantageous position. Second, charm the field to make the field chaotic and antagonistic for your enemy. Three, use hexes to finish off your opponent. The wizard battle style favors prepared defense.
Goblins wear armor and use swords and spears. They also dig intricate tunnels. Therefore their expected order of battle is to use tunnel combat tactics for defense. Elite heavy armor troops for offense. Magical armor to block the charms and hexes, blades and shields to block the transfigured conjurations. It is openly questionable how they would handle ground manipulating magics.
Centaurs keep range and fire arrows. For plains warfare, this strategy was dominant, see the popularity of kiting tactics in current games. Goblin troops can't reach them, or be hurt by them. Wizards can't reach them, but can block their path if they can see them, hence they switched to the forest instead of the plains.
Giants swing big club and magic does squat. Nuf said.
Dragons fly, breathe fire, and chow down. Nuf said.
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To properly wargame wizarding Britain, Death Eaters vs Harry Potter's Army, you need identify friend and foe, identify and prepare field of battle, muster and train troops.
Combat roles are:
spies to determine friend and foe
scouts to locate field of battle
engineers to prepare field
troops to fire hexes
The canon fields of battle are usually Hogwarts castle, Diagon Alley, The ministry of magic, someone's house. The only hard target in that group is Hogwarts castle. Homes are softly prepared. Diagon Alley and the ministry are porous complex urban environments that allow for easy entry, exit, and poor sightlines. Hence, everyone gets into little hex parties with each other at short range since no one got to prepare the field.
Therefore, knowing that battles occur with little warning, recommendation is to adopt goblin battle tactics: heavy armor shock troops with melee weapons. Get in close and gut them. Provide one time use shield to block curse and move fast. Armor only needs to block non 3U curses, those are what the shield is for.
No sightlines or range advantage makes kiting useless, running away a lot increases chance of stumbling on charmed or transfigured trap.
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Most of the story is spy battles. Who is friend? Who is foe? Are we sure? Can we flip them? What's their plan?
If that's not the conflict you want to write about, then you need watchtowers with comms to call in heavy troops to fight death eater raiding parties.