In my story, there would eventually be a final battle between a modern/high tech army from Earth and a coalition from another world. What are ways that the coalition can gain victory? Given that they possess a limited range of magic, a deep understanding of the modern army, enormous physical strength and overwhelming number?
This is my first question, so if I need to add or edit anything, please let me know.
Edit 1: Thanks for the suggestions for edit everyone! You guys have been really helpful despite the lack of information! Some members asked for the weapons and resources on both sides, and others of the likes for better answers. I will list them below
Edit 2: I've noticed that the question has been put "on hold" again instead of "Closed". It might just be an error, but in case it's not, I will change some stuff to avoid misunderstandings. One member pointed out that this question is too story-based, which I think might be true. Though I only wanted to know common strategies that can be used given the situation, which I will only use as reference, no more no less.
The Earthers know that the coalition is coming within two months, through portals which can be open anywhere at anytime, therefore they've been preparing to drive off the invasion (Evacuating civilians, fortifying buildings, etc.)
The coalition is well aware of how powerful the Earth army is, therefore, they don't wish for a long-term war (due to how difficult it is to transport food/water supply, troop transportation, etc). Earth had tried invade their planet before in the past, and they failed due to underestimating the coalition. And the coalition, though surprised of the Earth's god-like power, was able to stop the invasion while suffering heavy causalities. This is how they know of Earth's weapons and history, they also attained more information after interrogating Earth prisoners, such as how modern cities/buildings are very different from theirs and can be fortified in many ways, ETC.
Fighting: Fighting will take place mainly on military bases and cities, and where most evacuees will stay, the coalition army wants take civilians hostages to deter actions from the enemy. They will mostly ignore countrysides or jungles, where they assume are not important (can be changed).
For the Earth: every single type of weapons in modern 21st century.
Manpower: more than 300,000,000 strong, including regular, reserve & paramilitary forces, This is the result of every countries on Earth uniting to fight
Resources: Everything they have available, at this time, crops start to decline due to the planet dying out, and they only have 4 years of food supplies left if they use it all for this war, though water is still plentiful. So rationing will be really strict and might spark civil conflicts within the ranks or cities (The coalition isn't aware of this.)
For the coalition
Curved swords, spears, catapults, ballista (can pierce through tanks, Mangonel.
explosive crossbows: high accuracy, when not installed with explosive they can pierce through standard US body armor
cannon (something like a Madsen 20mm Cannon, though with longer reloading times) Their latest weapons, constructed based on their knowledge of the enemy. These are also very rare, as there are only about 300 of them at the time of the invasion
Armor: Leather (material can be changed if suggested) armor made of a giant species' skin on their planet, thick and strong to protect soldiers from standard US rifles.
Shields: Scutum-like Shields and Mantlet, both made of materials strong enough to deflect 50 cal and below
Calvary: Riders possess Bull-like creatures that can run at 150km/h at maximum speed, can destroy/crash through brick/concrete walls
Manpower: more than 900,000,000 strong, well-trained.
Resources: Plentiful in everything
Physical power: one individual soldier can be as strong as 10 professional human soldiers, due to the conditions of their planet
Magic: Summoning spirits that are only visible to the caster's eyes, these spirits can go through any physical barriers, the spirit can interact with any objects, and the maximum distance the spirits can be away from the casters are about 600 meters. The only way to get rid of them is to locate and kill the casters.
Communication: Flag, sound, smoke, light, rider signals to get messages across
Important notes: The coalition army looks exactly like humans, only with larger size (3-4 heads taller than average humans). And when they travel through portals to reach Earth, these portals are only 20 meters wide each, so it takes lots of time to bring all soldiers to the other side.
As of right now, I am open to any suggestions about various tactics that can be used regarding fighting in cities, either full-on frontal assaults or guerilla.