Are we able to see the patrol from here to check when is a good time to strike? Milo is able to cover that distance with ease so can attack at the best time to do so (which I would suggest is the midpoint between the last patrol passing and the next arriving so that everyone is maximum distance away.

Myrla crouches low behind the butte, her longbow drawn and her arrow nocked, the string taut as she keeps her sights firmly on the lone guard above the cave mouth. She squints against the fading light of the setting sun, its orange glow casting long shadows across the rocky terrain. Her eyes flicker between the guard and the cliffside, scanning for any sign of her companions’ progress. Her focus narrows again to the figure on the cliff. The details of its bony armor and tattered cloak are clearer now, unsettlingly regal even in its undead state. “Come on, you three,” she murmurs under her breath, willing her companions forward. "Stay sharp, Brix," she says softly to the gnome beside her. "If they slip up or that thing so much as flinches, we’ll need to take it down before it alerts the others." Her eyes dart back to the cliff’s lip, hoping for a sign that her companions are still on track.

Action: (if permitted) ready an attack on the lone guard, to be triggered by a sign of alarm in the guards demeanor.

Inge whispers, muffling her voice further with her long braid as Wadi might remember her doing from their hunts together in the White Waste.

"That thing looks a lot tougher than those other, ah... ghouls.  Like when it was alive, maybe it was some kind of soldier, with its armor and fancy blades.  Maybe even someone who used to be General LunaFooFoo's friends.  Do we charge it?  I know Milo can get to it and I think I can too."

(Inge has 35' move speed, so I'm assuming she can get within 5', or at least 10' of the creature, which is all she needs with her glaive's 10' reach).

"Thing is, it looks strong enough that it' won't go down in a couple blows.  Any way to stop it from, I don't know... sounding the alarm somehow?"

She studies the figure to discern any resemblance between the blades and armor it is wearing and the gear she has seen officers use in Lunafreya's army.

Wadi can help to distract it with the help of his fire spirt. "I can ask her to materialize next to it with an explosion of fire and then poof again with another explosion and appear some feet away. She can also help to move team members around as needed." 

Brix holds his breath as the others make their approach.

As they wait for an opportune moment, Inge scrutinizes the guard more closely. It is clear that this person, whoever it was previously, was not a part of the general's army. However, there is just a subtle feature and gait about the guard that says its military experience. It may not be a soldier of the general, but it is clear that it was a soldier. 

A brief peek at the patrol tells the group that there is no less perfect time to strike than now. The distance the last patrol group had covered from the cave entrance right at this moment will give the group quite some time before the next patrol group comes out of the cave. 

Adrenaline is beginning to course through Inge's veins.  Her voice is an excited hiss.

"Let's do this.  Milo, I'll follow your lead and Wadi, your spirit thingie can go on the other side?  Then it can move us if we get in trouble.  Gonna be a good fight.  I think whoever this was, it fought in some army.  Just from the weapons and stance.  Watch out for the smell..."

Inge's Initiative if needed:  20 (dirty)

As soon as Brix see the others engage he jumps from the hiding spot and starts husling towards where the action is.

Wadi moves forward as well

Init as need: 19

Myrla feels the tension building as she observes the distant forms of her companions edging closer to the lone guard. Her grip tightens on her bowstring, the arrow nocked and aimed with deadly precision. She can see the subtle shifts in their postures as they prepare to strike—a coordinated effort that, if executed well, might bring the guard down quickly. But then, to her disbelief, Brix breaks from cover and starts moving toward the cliff. Myrla’s jaw clenches, and she hisses under her breath, "Brix, couldn’t you wait?" Her heart races as she watches the gnome hustle forward, his boldness threatening to disrupt their plan.

"Well, if the guard notices him, at least it’ll serve as a distraction," she mutters to herself, adjusting her aim slightly. Her keen eyes stay locked on the guard, watching for even the smallest sign of alarm in its posture. If the creature so much as twitches toward Brix or shows signs of raising an alarm, her arrow will fly, precise and unforgiving. In the back of her mind, she reassures herself: 'I’ll catch up with him if I have to. Just let the others land their strikes first.' Her focus sharpens, and she steadies her breathing, ready for the shot.

Initiative: 16  

(She remains ready to release the arrow in case the guard shows a sign of alarm. Not sure if we play this with new or old rules, but in the 2024 rules if someone is surprised, they only have disadvantage on initiative rolls, not outright lose a round like in the 2014 ruleset.)

Milo runs forward (Init 19) but unfortunately he struggles to get a good attack in. He stabs with both rapiers, the first barely piercing, and the second failing to find purchase at all.

(1: Attack 23 to hit, for 5 damage. Attack 2: 8 to hit with a nat 2)

Assuming that the guard would show signs of alarm caused by Milo's attack, please find below Myrla's readied attack (in the spoiler). 

Wadi, as per the plan, summons his fire spirt appear next to the enemies with an explosion. 

 BA: The fire spirit then teleports back with another fire explosion

 Wadi would also move up as near as he can to melee range.

(Since others are posting their combat actions, I'll do so for Inge as well).

Inge's long strides (35' speed) nearly keep pace with Milo's innate quickness, her long dark braid lashing behind her like the tail of a big cat balancing its body at full sprint.  It takes all the willpower she has not to launch into a full-throated battle cry as she charges recklessly.  Briefly, she instinctively considers slamming into the sentry to grapple or shove it, but she realizes knocking it prone would hamper ranged attacks.

Once she is 10' from the sentry, directly behind Milo, and leaving room for Wadi's fire spirit and arrows from their other companions, she plants, pivots, and leverages all her forward momentum to swing her glaive wildly in a wide arc above Milo's head, using every bit of her strength.

Bonus Action:  Rage (round 1 of 10)
Action:  Reckless Attack on Sentry from 10' away with glaive (10' reach and enhanced by Brix's infusion)
Inge's Glaive+1 to hit (advantage from Reckless Attack):  20 (dirty)
Inge's Glaive+1 damage:  13 + 2 from Rage = 15 magical slashing

All attacks against Inge until her next turn have advantage due to her Reckless Attack.  But she has resistance to bludgeoning, piercing and slashing.

(Sorry if I was confusing. Meant to have Brix start out when the fight with the guard starts. But we can work with it as written. Lot of ground to cover. )

The guard stands firm and unmoving as Milo's attack pierces it. The first blade sinks in easily, but the second one catches on the guard's bony armour. The guard remains still even as Myrla's arrow sinks deep into its chest. 

{{Dex save: 18 and 19 (Saved) no damage if I recall.}}

It is not until Wadi's summon comes out that the guard moves. It grabs its own cloak and wraps it around itself, covering it completely from the burning effect of the summon. It turns toward a screaming Inge and catches her glaive with one of its hands. It appears surprised but amused as the glaive cleaves through its bony hand. 

"I told them that someone would come," the guard murmurs. Its voice is like the crushing of rocks -- guttural, hoarse and dry. "Good luck, children. You will need it going inside."

{{Initiative Order: You guys go first. The guard closes the round as I roll badly. As you get a surprise round. You get to go again immediately before the Guard.}}

(I believe Inge is first in Initiative with a dirty 20, so posting for her now):

Attacking the bone warrior recklessly, but in unaccustomed silence, Inge has a moment to wonder... told WHO that someone would come?  Why is this thing that was once a warrior not sounding the alarm?  Did he just call us CHILDREN?  Her long dark braid sways behind her as she pivots.

Inge continues to spin her glaive+1, infused with magic by Brix, slicing first at the torso with the blade, then thrusting at its throat with the haft.

Action:  Inge attacks the bone warrior recklessly with her glaive+1 from behind Milo (10' reach).
Glaive+1 to hit (advantage due to Reckless Attack):  18
Glaive+1 damage:  16 magical slashing

Bonus Action:  Inge flips her glaive+1 and jabs the haft into the bone warrior's throat. (Polearm Master bonus attack)
Glaive+1 haft to hit (advantage due to Reckless Attack):  26
Glaive+1 haft damage:  9 magical bludgeoning

Movement:  None - Inge holds her ground within 10' of the bone warrior, just behind Milo.

She continues Raging (round 2 of 10).  With the bone warrior within her 10' reach, she can take an opportunity attack if it leaves her reach.

Attacks against Inge until her next turn have advantage (Reckless Attack).  However, she is resistant to bludgeoning, piercing and slashing damage (Rage).

Myrla steadies herself, her sharp eyes locking onto the bone warrior now engaged with her companions. She draws another arrow, her focus unwavering as she adjusts for the shifting melee and the growing distance. The chaos of the battle—Milo’s quick strikes, Wadi’s fiery summon, and Inge’s powerful glaive attacks—creates a moving target that proves harder to pin down. Exhaling slowly, she releases the arrow, but this time it whistles past the warrior’s shoulder, embedding harmlessly into the rocky terrain beyond. Myrla grimaces, slinging her bow across her back as she rises from her cover. With no time to waste, she breaks into a swift run, following Brix toward the fray, her steps measured and precise despite the tension thrumming in her veins.

Action: hit 12 (assumed disadvantage due to distance, but straight shot would have been the same), for 9 damage if it does hit

Wadi moves to striking distance and strikes out with his 'magic'd' staff (11 to hit for 4 damage)

He swings back around and strikes out again (13 to hit for 2 damage)

With absolutely no features (other than action surge which is used, and second wind which is not needed yet), all Milo can do is keep stabbing his two rapiers.

Attack 1: 15 for 11 piercing.
Attack 2: 20 for 8 piercing.