Made this a little fluffed out, hope you don't mind.
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Gentlemen, welcome back to Regimental HQ. Sadly, you guys aren't going to be here much longer, we've got a situation from the front. Our assault on the Gryphon colony has finally slowed. Gryphon forces have been building up positions there, along the coastal roads, halting our advance. Combine that with the already rocky natural terrain and you've set yourself up one hell of a situation. The General has ordered us to swing out with the fleet and re-land on the shore, coinciding with his ground strike. Our target is the beachhead here, at the city of Mirsina. Resistance is going to be tough, but if you get a beachhead and take at least a block of the city, we'll be able to reinforce you. Good luck.
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Terrain: A small recreational beach on the edge of a small coastal town. About a hundred meters on shore, the beach ends, opening up to the town. The town itself is heavily fortified, but bombardment has reduced some buildings to rubble.

Objective: Take and hold the beach and at least a single city block of the town.

DRM Troops (Player)
73 DRM Riflemen
20 DRM Troopers
Hero: None

Gryphon Troops:
30 Gryphon Heavy Infantry
70 Hippogriff Light Infantry
20 Gryphon Mercenaries

499402

Divide riflemen in groups of 36 and 37 men, keep the troopers as they are.

Have the navy bombard the beach to create craters for cover and then deploy the soldiers.

The men approach the beach in silence. The cannons roaring in the distance, the sea breeze blowing from the seas.

Salt and worry was in the air, fear and excitement overflowing the atmosphere as the boats approached the beach.

Enemy soldiers could be seen flying over the ruined buildings readying their defenses and preparing for battle.

The soldiers hit the sand, boats stop moving and dozens of warriors rush to take the beach, to challenge death and meet destiny.

One group of riflemen charges ahead while the other takes cover among the craters, offering cover fire. The troopers remain out of combat until further orders, staying at the back craters.

The enemy is numerous but lacks missile troops and most of their soldiers are pitiful low class slaves and conscripts. The riflemen have no trouble securing the beach.

Enemy gryphon mercenaries fly over the sands and shoot their crossbows as they dash past the bullets of the men, but there are too many rifles and too few of them. The gryphon mercenaries lose half their unit before retreating back to the city.

Enemy light infantry attempts to overrun the soldiers are met with horrifying losses on their ranks, dozens litter the fields in seconds and the rest flee for their lives.

All enemy heavy infantry that accompanied them was forced to retreat as their numbers were outmatched by the sheer amount of riflemen.

The enemy army is in shambles, and I have lost few soldiers. The riflemen press into the city, securing building after building. Leaving no stone unturned.

Enemy resistance becomes fierce as soon as the soldiers enter the town. No longer able to shoot at gryphons from afar, the men find themselves at a complete disadvantage.

Not being able to press forward, I order a tactical retreat and reform the soldiers.

Enemy units regroup and the battle takes a turn for the worst. Holding the knowledge of their own city and with heavy infantry units, the gryphons entrench themselves inside the buildings and await for my men to approach.

Unable to face them on equal terms, I fortify the first few blocks I managed to capture after the battle on the beach and hold it.

Awaiting for reinforcements.

499402 That's not very useful infomration, what exactly are the Gryphon fortifications?

499785
Sandbag clusters, bunkers, metal fortifications, you name it.

499785

My little Berserk, information is delaying!

499907 Sorry, gonna need more detail than that.

First of all, let me say that the way you put the briefing fits perfectly with this soundtrack. It usually plays behind a briefing scenario in AC05. The first part, I read in the commander's voice, and the layout in the announcer voice. So perfect.
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Anyways, I will use the exact situation you gave here.

Commands given: Split, Charge

The 73 Riflemen and 20 troopers will be split into three groups of 24-7, 24-7, and 25-6 respectively. They will attack in a three pronged attack on the beachhead, forcing enemy units to split up.

All the Griffon Light Infantry desert when the other warriors go in to combat the DRM force. Why? Because there are no Enforcers around.

This leaves 20 Mercs and 30 Heavy Infantry to the bullets of my landing force. The teams have the firepower of the riflemen, and in the need, the skill of a substantial amount of Troopers for each group.

Result: All enemy combatants engaged killed. Beach and town secured.