Alex quickly tracked down the first crew of pirates to beat and casually drew his sword. Then, he went into a stance, his sword behind him at an angle where he had a big range to swing.
A beat later he turned forward, and in that motion his hand holding the sword swung towards the enemy ship with such force that a purple slash was released from his blade, flying at unprecedented speeds and cleaving through the main mast of the ship, injuring a few pirates who were lounging around on the deck in the range of the mast. As it fell, it got a few limbs nicked, and a few were injured more seriously than a mere scratch along the arm or a leg.
The captain of the enemy pirates quickly got the whole crew out, and they had a standoff with Alex and his Marines. Sei grinned, Boomer procured explosives in both hands, Aika, the cook, sharpened her knives, Elle Von Durne cocked her handgun with a smirk on her lips, and Alex let a huge shit-eating grin appear as he subconsciously released some Conqueror's Haki, his emotions affecting his tight hold, but Alex noticed soon and reeled the output in, his grin never fading as he spoke one sentence to his crew: "Let me solo them."
The pirates were baffled. This single Marine rear admiral wanted to solo them? They hid malicious smirks as they thought about how scared this guy will be when they beat the crap out of him, maybe put his head on a pike—yeah, definitely on a pike.
The battle was swift. Alex relied on his immense speed, reaching low admiral level, to inflict quick yet decisive slashes with his swords, and if that wasn't enough, a second one would finish the job. Any and all efforts on the Gang Gang Pirates' side were evaded or completely countered by Alex, who used his Observation Haki to anticipate every move made by his opponents.
Within minutes, he had beaten the crew of 50 New World Pirates and apprehended the captain of the Gang Gang Pirates. He wanted to cash in a bounty to buy some ingredients for Aika; she loved to cook, and he loved her cooking.
After wrapping things up, Alex went about hunting down and defeating the easy- and mid-level crews. He had made sure to ask Brook whether or not he had eaten a devil fruit, receiving a 'yes, but it didn't give me any powers.' I nodded and quickly killed both him and his crew. Brook shall be born again, yohohohoho. Alex did feel bad, but he knew that Luffy and Brook both needed this one.
With that done, Alex now had to hunt down the following crews:
Demon Pirates (mid-hard-level threat):
Savage, bloodthirsty fighters who use brutal strength and monstrous Zoan devil fruits to overwhelm opponents.
Robo-pirates (mid-hard-level threat):
A crew obsessed with technology and cybernetic enhancements; part pirate, part machine.
Cook pirates (hard-level threat):
A vicious group of cannibal chefs who believe in "cooking what you kill" and are infamous for hunting enemy crews and civilians like prey.
Whitebeard Pirates (Rocks Pirates branch) (impossible-level threat):
An elite branch of the Rocks Pirates, commanded by the rising monster known as Edward "Whitebeard" Newgate, each member a terrifying powerhouse in their own right.
Alex had a few plans for each crew. For the Demon Pirates, he'd ask Boomer to whip up some Kairoseki/Sea-prism-stone bullets or weapons like a sword in order for them to be unable to transform if they proved too troublesome to deal with the old-fashioned way.
As for the Robo-Pirates, he would ask Boomer to make some sort of EMP or something that just stores a wave of electric energy and can release it in a burst to disrupt the robo part of themselves, probably stunning them and allowing enough time for a takedown, just in case.
The Cook pirates simply have to be defeated, and he'd have to scope out the Whitebeard crew himself. If they were too strong, he'd power-level for a bit, and then he'd return. After all, he himself hadn't gotten any timeframe as to when to complete the mission, plus he had already brought in a few of the ones on the list.
With the planning done, he relayed his ideas to Boomer, who clapped him on the shoulder and said that he was a goddamn genius while he scrambled around for material to start working on the stuff he had asked for.
After a little while purely spent training and doing nothing else, Alex and his band of Marines finally made it; they had tracked down the savage crew, the Demon Pirates.
Their bounties were all in the same few hundred million to 1 billion range, making them quite a fearsome crew, definitely a tricky opponent, but Alex had another idea: since they were quite a ways away, he wanted to try Grandpa Garp's favorite activity: chucking cannonballs at pirate ships till they sank.
His crew watched with growing stupefaction as he took out about 10 cannonballs and placed them in a neat pile and took a spyglass, checking the distance with a quiet precision and focus.
Aika stepped closer and tried to dissuade him with her sound logic. Well, Alex wasn't sound, and he definitely didn't work with logic when it came to stuff like this.
"Uhm, Alex-san, I d-don't think cannonballs are to be used in the way I think you want to use them."
He gave the spying glass back to the comrade he asked it from and did a bit of stretching, then he did a couple of light toss-ups with the cannonball, getting a feel for it, and then he stood at the bow of their ship and positioned himself like a pitcher, and with a grunt of exertion, the cannonball flew off with a bang that signified the breaking of the sound barrier.
Boomer was the only one who spoke before it happened: "What the-"
The cannonball whizzed through the air, actually igniting on fire and crashing right into the Ship of the Demon Pirates, a warning shot, so to speak.
The Demon Pirates turned and spotted a Marine ship far away, but it was too late; a second ball hurtled toward them. This time it was a bit more black than before, a clear sign of Haki being involved. Their main mast was destroyed, and the cannon next to the mast was knocked into the sea as the mast tumbled down.
They were now stuck, and Alex would make sure to get them all.