While there already is a compbuilding guide, courtesy of Guroteske, it has occurred to me that many people still commit the same mistakes over and over again.
Here is a compilation of those issues, to be expanded as they come up during my #help activities.
Prinz Eugen, Illustrious, Victorious, Atago, Belfast... the list of units with a shiny gold finish and without the quality you'd expect from it goes on; conversely, Langley, Ranger, Nevada, Leander and more all provide great examples of excellent units despite their bottom-of-the-barrel rarity.
Don't get scammed by golds, don't scrap your grays before you're sure they are no good.
The question you should ask yourself isn't "which class of ship do I need", but "what is this ship good/bad at?" and "will it work with the others?"; that is because every faction has its own spin on the various hull classes, and often enough there are multiple spins in the same nation, making sweeping generalisations hilariously inaccurate.
Laffey (USS DD) and Ayanami (IJN DD) are nothing alike; and neither are Laffey and Maury (also a USS DD).
Consider each unit its own thing, check its statline/skillset, and seek to answer those two questions above.
You save Oil by using 1 MLB backliner, possibly a BBarrager (BB/BC/BM with a barrage); and 1 MLB vanguard, possibly one with a self-heal (like Cassin/Downes/Phoenix).
It'll be cheaper and more effective.
While a 'full healing fleet' sounds like a fun meme, it isn't; the non-event self/healers you get, as sturdy as they are, are also lacking in DPS and Azur Lane is slanted significantly towards offence, not defence.
Turtling doesn't win you games in RTS and it certainly doesn't here, as later on doing that means the backline will get blown to bits or planed to bits; offence truly is the best defence, as never forget, dead enemies deal no damage.
- This does not matter for W13, as the sheer amount of pressure both the backline and frontline are subjected to makes taking 1 healer and 1 self-healer an appealing prospect.
Related to the point above. Treat boss fights as DPS checks, as your healing does not have the potency nor the speed to match the enemies' DPS and this will only become more apparent the farther you progress; the only way you get out of a lategame boss fight is by killing the boss before it kills you, and healers by and large trade their DPS for support.
The point is thoroughly covered in the Tier Guide, but tldr; Repair Ships' healing is pitiful compared to that of proper healers (Unicorn/Shouhou/Ryuuhou/Arizona/Perseus) and bring no DPS to boot, on account of having no planes nor surface guns to fire. What they do have is two twin AA gun slots with a good (for backliners) 100% Efficiency and three auxiliary slots to fill with AA Radars.
This makes for some impressive AA dakka, but before you run off using them in W12 - remember it also comes at a steep cost in support and DPS.
Refrain from using them unless you're desperate, and even then go for Vestal, at least she can rescue someone from near-death.
DO NOT PUT A CARRIER AS YOUR FLAGSHIP. (unless you it's CL-armable / you're bringing 3 carriers anyway / you're bringing 2 USS BB pre-W12)
Carriers lack the HP, the armour and - more importantly from W1 to W11 - the secondary guns needed to endure incoming threats.
Battleships and Battlecruisers all do, and often enough also happen to have barrages which will be most effective (if at all) only when launched from the flagship position; barrages that work from the sides only come with a handful of big guns, covered below.
- Zeppy/Bearn/Eagle are an exception, as they can be armed with defensive CL guns.
- 3CV/L comps are another exception, in which case you'll want the toughest carrier as flagship.
- 2BB 1CV USS comps are an uncommon sight but can be quite effective, and the reason for the flagship being in the middle lies in the DD secondaries carried by those American barrage-less Battleships; if they DO happen to have a barrage, however, you'll still have the BBarrager as the flagship as you'll benefit more from the barrage than from the improved coverage.
Related to the point above. The vast majority of proper Battleship/Battlecruiser/Monitor barrages fire directly forward and will neither aim nor curve towards enemy units; this means that to operate properly, the BBarrager will have to sit in the flagship slot, and that if that is not the case most of the barrage will splash harmlessly into the water (or all of it, in Pennsylvania / Washington / Alabama's case).
If you insist on taking two BBarragers at once, make sure to have the stronger barrage at the flagship spot and the weaker one to the sides; alternatively, consider the following...
- Georgia and Amagi have strong barrages with two main components: an aimed one that can strike whatever is on the field, and a frontal one that can strike only in front of them. In both cases, the two components roughly make up for half of the respective barrages' damage.
- Kirishima's barrage is fully aimed, with the Sanshikidan shells attempting to head for the nearest target; that said it's Sanshikidan, the damage will be scattered over a large area and in planes' dumb faces.
- Tosa is also an exception, as her barrage always launches from the flagship slot regardless of whether she's in there or not. This CAN be an issue, as in some very specific cases you may want to barrage at something that's not in the centre (eg. 13-4)
- Gascogne's barrage works best when she's in the flagship position, but if she happens to sit on the sides, it's wide enough to strike the entire upper/lower half of the screen.
- Odin sits in front of the flagship no matter where you put her (and if she IS the flagship, she'll just sit in her usual spot in front of it), meaning her barrage is always centered.
If you've fixed all these issues and you still find your fleet's performance to be lacking, the possible causes can be narrowed down to a subpar gear selection/enhancement (learn how to fix it here), being underleveled compared to the enemies (level up), having employed incorrectly your units or having taken too many bad ones (learn how to fix these here).