NOTE: I'm uncertain of some conditions. I'm writing this guide just from reading the forums and what I remember after 100%ing the game; as such, I am not sure of the exact conditions for many of the achievements. Please help me improve the guide.

War of the Human Tanks: ALTeR has two campaigns: the main campaign, of which there are 8 episodes, and the Great Cavern of Fuji, which has 101 floors, and is accessed through the menu in-between missions in the main campaign. There are also several unique free battles against Megumin and the Daihon'ei Corps that are accessed through the free battle menu. To obtain all of the achievements in the game, it is necessary to beat the main campaign 3 times, clear all 101 floors of the cavern, and beat several free battles; this took me 43 hours in total. Some miscellaneous things to note when playing through the game:

• There is a Japanese wiki for this game with all the gameplay information you could possibly want (at least, I assume, as I can't read Japanese) : http://wikiwiki.jp/human_tanks/ .
  
• Some campaign and cavern battles take you immediately to a game over screen when lost, while others do not. In the campaign, battles 2, 4, 6 and 6.5 may all be lost and the campaign will continue (losing episode 4 provides an alternate episode 6, otherwise there are no significant changes to the route). In the great cavern, losing most early battles will take you back to the intermission screen, while some later battles will take you directly to a game over (be careful with your saves).
  
• The vast majority of the cavern levels (>8) are locked until the main campaign has been completed. Combined with the fact that the final campaign battle becomes somewhat more difficult after you beat 150 and 300 tanks before attempting it (see "Shot the Albatross" below), this means that it's probably best to wait until new game+ to attempt it.
  
• As the game advises you, it's best to start off developing either royal or imperial tanks, but not both, because they are extremely similar. They are not the same, though, because some advanced tanks require you to have built one or the other (including the crucial Koharu, which has a God Eyes as its set bonus and requires imperial tanks).
  
• Enemy control tanks will prevent you from using any equippable modules ("set" modules are not affected) whenever they are deployed, which includes the final main campaign episode and most cavern levels. This includes Traffic Insurance/Tank Life Insurance, so don't bother using them on any such level. Destroying the enemy command tank does not reactivate your modules, and neither does deploying your own command tank or wearing helmets of any kind.

Cardboard Warrior Ruru
Completed episode 1


Devastation
Completed episode 2


Trainwrecked
Completed episode 3


The Obligatory Hot Spring Episode
Completed episode 4


Blood in the Snow
Completed episode 5


Super Patrol
Completed episode 6


Most Wanted
Completed episode 6.5


Overwhelming Odds
Completed episode 7


Altered Future
Completed Episode 8


You will obtain these achievements by winning every battle in the campaign.

Treasure Hunter
Completed episode 6 (Alternative)


Losing either episode 2 or 4 will give you an alternate episode 6 with this achievement.

Half Human
Half Human Tank


The difficulty of the final battle depends on the number of tanks you have destroyed in the current march (visible on the intermission screen). If it is 149 or less, Albatross appears as a 2x2 tank. If it is between 150 and 299, Albatross appears as a 3x3 tank. If it is 300 or more, Albatross appears as a 3x4 tank. Beating the final battle with >299 kills grants a playable Elle, as well as this achievement.

Shot the Albatross
Big, Bigger and...?


Obtained by playing through the campaign three times at each different kill level.

How Deep Does This Go, Anyway?
Answer: deep
[Floor 15]

A New Enemy
A fourth force has appeared...?
[Floor 25]

Mystery of the Underground Civilization
Explored ancient(?) ruins
[Floor 35]

Dreadnought Class
Defeated the mightiest train of them all
[Floor 57]

The Dark Corps
Explored a world of illusion
[Floor 70]

Ultimate Shadowboxing
Defeated the Illusion System
[Floor 88]

The Gymnast
Defeated the strongest gymnastic formation
[Floor 92]

Trimmed
Obtained the Nail Clipper
[Floor 96]

I'm Not Homeless, I Just Like It Here
Settled Ruru into her natural inhabitat
[Floor 97]

It's Orange
Obtained the Orange
[Floor 99]

Giant Killing
Defeated the Juggernaut of Void
[Floor 100]

White Future
Reached the end of illusions
[Floor 100]

An Exercise in Futility
That'd better be the last one...
[Floor 101]

These achievements are obtained by progressing through the Great Cavern of Fuji, listed in order of floor obtained. Note:

• Floor 24 rewards a Cat Ears tank and floor 73 rewards a fuse module which are required for "Lethal Cuteness" and "I Scream" respectively (see below); don't destroy them. If you do, you'll need to play through the floor again through the cavern menu (NOT the free battle menu).
  
• You are given the option to skip floors 92 ("The Gymnast"), 96 ("Trimmed") and 99 ("It's Orange").
  
• Though they have the same sprite as the invincible units, the 3 command tanks on floor 88 are not invulnerable (see map below).
  
• For "I'm Not Homeless, I Just Like It Here", choose to equip the cardboard box on floor 97.
  
• The boss unit on floor 99 may not be hit unless the status of your army is "cornered" (have fun!).
  
• Floor 101 ("How Deep Does This Go, Anyway?") is only accessible after starting a new game, because floor 100 ends the current playthrough. One of the enemy shock tanks is invulnerable until your army has "dominant" status.

Ends Justify the Means
We won't forget your sacrifice


Win a battle by destroying the enemy command tank and your own unit in the same attack. I've been told slightly conflicting things about whether there are other ways to trigger this achievement by destroying some combination of your own and enemy units; it shouldn't be much of an issue because getting the achievement as stated above is simple, but if you can confirm you got it another way, please let me know in the comments.

Total Control
Enemy technology has been contained


Destroy 50 modules with control tanks. Control tanks can only be developed after clearing cavern floor 5.

Yakiniku
Oh, yeah!


Clear free battle 014 or floor 12 of the cavern.

Strayminator
2.0.0.


Defeat 200 stray tanks. This includes all battles where the opponent is listed as "Stray Tank Corps", including episodes 4 and 6 as well as most early cavern levels.

Sniper Elite
For Excellence in Ranged Combat


Win a battle, destroying the enemy command tanks with only snipers.

Faster Than Light
It was over before it began


Win a battle with a speed bonus. The simplest way to do this is to deploy a sniper with a haste module on free battle 001, and snipe the enemy command unit.

Golgo 13
The lone assassin left no survivors


Destroy all enemy units on a map, attacking only with one Tougou. Use the target area +3 module.

Twin Win
Together, the two are unstoppable!


Win a battle after only deploying one Ruru and Rara. Rara is obtained after beating the main campaign with <300 kills.

Lethal Cuteness
Meow?


Win a battle using only one Cat Ears. Cat Ears is obtained after clearing cavern level 24.

Strangelove
Learned to Love the Bomb


With a shock tank, destroy every enemy on the map. Most easily done with a radiotoxic bomb or target area +3 mods.

Beat Them to the Punch
Let your fists do the talking


Win a battle attacking only with fighting-type shock tanks (a la Pokari). Fighting tanks can only be developed after cavern floor 35.

Daihon'ei Corps
The crew is back together


Deploy (any versions of) Heshiko, Miou, Koharu, Masamune and Touka in a battle. These units are unlocked by battling the Daihon'ei Corps in free battles 016, 017, 061, 144 and 115. While clearing all these battles will get you the required units, Juff's guide explains the special conditions required to obtain alternate versions of these tanks.

I Scream
It's scarier than the enemy...


Win a battle deploying only one Mirage, available for development after clearing floor 73.

Foil Hats
Good for preventing hostile radiowaves... maybe?


Win a battle with every unit equipped with the aluminum foil module. No, it does nothing besides reduce your transmission speed.

Helmaniac
Gotta have all those helmets!


Develop every standard imperial tank as well as the imperial control tanks available from cavern floor 5. Advanced tanks from the lower floors of the cavern aren't required.

A Closetful of Hats
If only they wore helmets too...


Develop every standard royal tank as well as the royal control tanks available from cavern floor 5. Advanced tanks from the lower floors of the cavern aren't required.

G-Ghosts don't exist!
It's rather cute than scary, though...


Clear floor 35 of the cavern and then amass 2222 total kills across all playthroughs.

And finally...

ALTeR
Completed every achievement in War of the Human Tanks - ALTeR

Challenging battles in this game are neatly broken down into two types, based on whether the enemy deploys a control tank or not. If the enemy does not deploy a control tank, it is usually a matter of deploying a Koharu tank with a haste device to gain full visibility of the battlefield, then loading and using some combination of the target area +3, shooting range +3, diet cola and haste device modules to overpower the enemy. It should be noted that the Koharu tank is useful even when an enemy command tank is deployed because it has god eyes as a set bonus and set bonuses are not affected by control tanks. Koharu is available after beating free battle 016. An example of effective module use is floor 19:

An otherwise difficult fight made trivial by using a Francoise with a target area +3 module.


For battles where the enemy does deploy a control tank, it's usually best to deploy a line of Chappys (early cavern) or Ludwigs (late cavern) along the front lines, along with a command tank, Koharu, a few sniper units and a control tank or Parfait where required. An example of this is floor 88:

The initial setup has a line of Ludwig tanks to serve as fodder for the immortal frontline Toukas. The three vulnerable command Toukas (circled) are later taken out by Pokari and Misora.

I'd like to thank both the developer and translator for this charming game. Seriously; keep up the good work!

I'd also like to thank the following users who posted information about many of these achievements on the forums or in the comments:

Juff
Thallassa [原子力潜水艦]
[Øk] chai
Promestein
Anubis
TeInaba
ElmntNinja
Red Katyusha
Konato_K
Brony Down Under™
Vapor Dosh
Diasuke