The following catastrophe is what happens when an average player tries to make a guide. Take everything I say with a grain of salt.
A ship almost universally derided (and rightfully so) as one of the worst ships in the game.
Yet somehow... She became my most played ship.
So, 150 battles later, I'm writing a guide for the new player who has to slog through this ship like so many others have before.
Super Quick Condensed History:
The twelve Mutsuki-class destroyers were built from 1924 to 1927. They were an improved version of the previous Kamikaze-class destroyers, introducing triple 24" torpedo tubes. All twelve ships were named for traditional poetic names of the months of the year of the Lunar Calendar. Throughout World War II, the ships received different modifications, such as the loss of one or two guns and torpedo mounts, providing more deck space for cargo and AA, facilitating their role in "Tokyo Express" transport missions. None of the ships survived World War II.
9 500 (stock) 11 300 (upgraded) hitpoints
Upgraded, Mutsuki has 11 300 hitpoints. This is far on the low side for a tier 5 destroyer, only more than Minekaze and Kamikaze (+clones), and equal with T-22.
Speed and Maneuverability:
Maximum speed: 37.5 knots
Turning circle: 550m
Rudder shift: (3.2 stock/2.1 B-hull)
For a tier 5 destroyer, you're middle of the pack. You're half a knot faster than Nicholas, Gremyaschy, Jianwei, and you're quite a bit faster than T-22. You're slower than Minekaze, Kamikaze, and far slower than Podvoisky.
As for maneuverability, you're equal to Minekaze and better than all the other tier 5 destroyers. Realistically, you're second among the tier fives because Minekaze's higher top speed means that Minekaze will be able to complete her turn quicker than Mutsuki.
2x1 120mm/45 3rd Year Type (stock)
2x1 120mm/45 3rd Year Type Fancy Mount (upgraded)
Range: 8.1km (stock) 8.9 km (upgraded)
Your gun armament is pathetic. That's the only way to describe your guns. Stock, Mutsuki has weaker gun power than Isokaze a tier lower, with the same fire rate (12 seconds), more AP alpha (2000 vs 1800), and a higher shell velocity (825m vs 660m), but only half the gun mounts and a slower turret traverse. You would struggle to best a tier 1 in a gunfight. You can't keep your guns on target during a turn. Simply put, your stock guns are useless.
Luckily, there's hope! You can upgrade your 120mm/45 3rd Year Type guns to... 120mm/45 3rd Year Type guns. "What's the difference", I hear you ask. Well, the guns are now mounted in large late-war mounts. Your gun power, in my opinion, now better than Minekaze and Kamikaze. Better, however, doesn't mean good, or passable. Your gun power is still bad. It's just less bad now. Your rate of fire more than doubles, from 12 seconds to a brilliant (for an IJN DD anyway) 5.5 seconds. This is the third fastest out of any Japanese destroyer, only behind Akizuki and Harekaze, although Yugumo and Shimakaze can still attain a higher rate of fire after upgrades due to the high tier reload modification. Your turret traverse increases as well, from 25 seconds to 18 seconds. This is now enough to keep your guns on target even during a close range knife fight, and faster than almost any other Japanese destroyer. The guns are slightly more accurate now as well, from 82m to 76m maximum dispersion.
These guns, individually, don't look half bad, don't they? So, what's the problem? Well, you still only have a grand total of two guns. Mutsuki has the dishonour of having the fewest number of guns on any ship in the game other than aircraft carriers. They also have meh gun arcs forward but pretty good arcs to the rear. Discounting a low health destroyer, the only ships at your tier you can hope to defeat barring a lucky torpedo strike will be:
Minekaze and Kamikaze: very poor turret traverse means that you can defeat them in a knife fight with your superior turret traverse. Watch out for torpedoes, especially Minekaze's fast loading fish.
Podvoisky and Gremyaschy: poor turret traverse that cannot keep up with quick maneuvers means that you can defeat them in a knife fight with your superior turret traverse. Watch for high alpha potential of 130mm guns and torpedoes.
T-22: pitiful gun power due to weak 105mm guns and German HE. Watch for high rate of fire and torpedoes.
Against Nicholas or Jianwei you may as well not even try. The best choice when up against another full health destroyer will almost always be to pop smoke and run away.
2x3 Type 8 mod. 3 torpedoes
Right, we're getting into the meat of the matter. These torpedoes will form the bulk of your damage numbers. They have 8km of range and a 73 second reload. They offer 14 600 damage and make their way towards victims targets at a reasonably quick speed of 63 knots. Pretty good, eh? These torps starting to make you wonder why this ship is so hated? Well, here's why. First off, you have two triple mounts, rather than three twin mounts like Minekaze or Kamikaze. This means that they have a longer reload than twin mounts. Also, there will be larger gaps between torpedoes when you launch. You also have less freedom with torpedo usage. In Isokaze or Minekaze, you can launch four torpedoes and keep two for emergencies, or use them to cover more ground. You can't do that as effectively with Mutsuki. This is the first Japanese destroyer torpedo to be hit with the longer detection ranges common among Japanese ships, with 1.6km surface detection as compared to 1.4km surface detection or lower compared to other tier 5 destroyers.
6x2 25mm: 30dps @ 3.1km (stock)
4x1 25mm, 6x3 25mm, 2x1 120mm: 44 dps @ 3.1km 6 dps @ 5 km -- 50dps total (upgraded)
You know, there is a reason two of Mutsuki's main guns were removed. That reason is to cram as much AA as possible on the ship.
For a tier 5 destroyer, your AA isn't half bad. This is one dps better than Nicholas, and while Nicholas gets defensive fire, you can bring all your DPS to bear at a longer range than Nicholas can. Only T-22 has more raw DPS.
Your upgraded guns are dual purpose, which was why they gained all that traverse and fire rate. One slight problem, though. Your 120mm guns reach out to 5km. Your air detection range is 3.1km. This means that you will need to turn your AA off (press P to turn AA on and off) to keep stealth until aircraft come within AA range, where you can let loose and watch planes fall (slowly perhaps, but they will, which is already better than a lot of other destroyers).
These four 25mm triple mounts replace two of your 120mm guns
6.2km surface detection
3.1km air detection
2.2 smoke firing detection
You are really stealthy. Really, really stealthy. You can creep undetected to fantastic positions to launch torpedoes at the edge of stealth. You can keep other destroyers lit while staying in stealth yourself. When fully rigged for stealth with concealment expert and camo, you will have 5.4km surface detection and 2.8km air detection. You are stealthier than all except for Minekaze and Kamikaze, which match your detection. This will give you enough stealth to contest caps because you will be able to spot destroyers entering it to give you enough time to run away (except for Minekaze+Kamikaze, but let's be honest, they're not going to want to gunfight anymore than you do).
Make sure to equip camo, any camo that offers a concealment boost.
Captain Loadout and Upgrades (First 10 points):
For one point skills, I recommend Priority Target, so you know how many players want you at the bottom of the ocean. For a second point skill, I recommend Last Stand, basically a standard skill for destroyers, because of how easily your rudder and engines are knocked out. Torpedo Armament Expertise to reduce your torpedo reload to 65.7 seconds is probably the best skill for three points, because at tier 5 it's unlikely that the extra consumables offered by Superintendent will be put in use often. Survivability Expert probably isn't needed because that's more needed to survive gunfights, something you should do your best to avoid. For four points. Concealment Expert is a no brainer.
For your first upgrade slot, I recommend Main Armaments Mod. 1 to make your torpedo mounts more durable. For your second slot, Propulsion Modification 1 because while you are still reasonably maneuverable with Last Stand and a damaged rudder, you will take a rather big hit in speed. Aiming Systems Modification is the only real choice for slot three, unless you want to set up an AA troll build.
Sierra Mike to boost your speed to 39.4 knots
Juliet Charlie flags to remove the chance of detonation
Juliet Whisky Unaone to improve your flooding chances
November Foxtrot to improve your consumable reload time
November Echo Setteseven if you really want to be an AA troll that badly
Mutsuki is a ship that will teach you to not pick fights with other ships unless you're sure you can win and to stealth torp (if you don't already know that's launching torpedoes from outside your detection range. Turn on the detection rings on the minimap). The guns are individually fine but are handicapped by having only two guns and poor forward firing arcs. The torpedoes have large detection ranges but otherwise aren't bad. Your AA is decent enough and might keep a low tier aircraft carrier off you but don't expect to be some AA escort. You are stealthy and reasonably fast, and you should use that to keep out of trouble. I actually rather like Mutsuki, oddly enough. I guess she just clicks with me or something.
Good luck with this ship, and may your grind be quick!
Kancolle Pic to trigger people