Welcome to Killer Instinct Classic. This game is sorta rough. Most of the achievements are not that difficult but do require a degree of patience. Beating the game on a single credit will be annoying though as this game, even on easy difficulty, cheats by facing you off against extremely difficult opponents, especially if you are not a veteran of Killer Instinct. The achievements break down is as follows...
12 achievements are for beating the arcade ladder
11 achievements are for winning matches with characters
2 achievements are for performing humiliations
2 achievements are for performing ultras
1 achievement is for continuing arcade ladder after dying
1 achievement is for winning a match in every arena
1 achievement is for unlocking everything in the game
This walkthrough breaks down the achievements by character and following it in order will nab you all 30 achievements for 1000 gamerscore. Enjoy!
There are 2 versions of this game you can play: Classic version (1.4) and Final version (1.5) You will want to play 1.5 as it is easier to perform certain moves and the "play as Eyedol" code works better in it.
The control settings for this game are relatively standard when compared to most fighting games. If you have never played one before I will attempt to explain below.
You have both high attacks and low attacks. High attacks are also referred to as punches and low attacks are referred to as kicks.
Quick punch = QP = 
Quick kick = QK = 
Medium punch = MP = 
Medium kick = MK = 
Fierce punch = FP = 
Fierce kick = FK = 
You are free to change these buttons up to your liking in the game's options menu.
Also you must, in order to perform an ultra combo, first do a lead-in combo. The best lead-in combo for any character is to jump towards the opponent with a QK, followed by a standing QP. Input your ultra combo immediately after the QP. This must be started when the opponent is near death on their second life bar and the ultra part of the combo can only be performed when the opponents health bar is flashing red. Half of the characters ultra combos require you to be holding a certain direction for a bit before finishing the combo. It is perfectly acceptable to jump towards the opponent with your QK for the lead in and start holding the direction required before you hit your QP. That will always give you enough of a charge to input the rest of the combo.
Additionally, in order to perform a humiliation, you must be on your first life bar when you deplete your opponent's second life bar. The screen will flash red and you will hear a voice say "Danger, Danger" after which you will want to input the combo about half a screens distance away from your opponent to perform a humiliation.
Lastly, here is a list of all characters ultra combo and humiliation moves:
CINDER:
Humiliation: Back, Back, Back, FK
Ultra Combo: Forward, Forward, FP
SABREWULF:
Humiliation: Forward, Forward, Forward, QP
Ultra Combo: Charge Forward, Back, QK
RIPTOR:
Humiliation: Down, Down, Forward, Forward, FP
Ultra Combo: Charge Back, Forward, QK
SPINAL:
Humiliation: Back, Back-Down, Down, Down-Forward, Forward, FK
Ultra Combo: Back, Back-Down, Down, Down-Forward, Forward, FP
FULGORE:
Humiliation: Back, Back-Down, Down, Down-Forward, Forward, MK
Ultra Combo: Forward, Down, Down-Forward, QP
TJ COMBO:
Humiliation: Down, Down, Down, QP
Ultra Combo: Charge Forward, Back, FP
JAGO:
Humiliation: Forward, Forward-Down, Down, Down-Back, Back, MK
Ultra Combo: Down, Down-Back, Back, QK
THUNDER:
Humiliation: Down, Down, Down-Forward, Forward, QK
Ultra Combo: Charge Back, Forward, QP
GLACIUS:
Humiliation: Forward, Forward, Back, QK
Ultra Combo: Charge Back, Forward, FP
ORCHID:
Humiliation: Forward, Forward-Down, Down, Down-Back, Back, FP
Ultra Combo: Charge Back, Forward, MP
Feel free to jump ahead to the character you need progress for. This game will be a grind as not only will you will need over 500 total wins split between the game's characters, but you will also need a supreme victory with each and even a perfect win for Riptor, Sabrewulf and Spinal. Note that the achievement description says to do this in arcade mode but you CAN do it with a second controller.
Jago
TJ Combo
Orchid
Cinder
Sabrewulf
Spinal
Glacius
Chief Thunder
Fulgore
Riptor
Eyedol
Misc.
JAGO
We will start our journey with Jago. Start an arcade ladder and knock your opponents health down so that it is flashing red on their second health bar. Do a lead in combo (a great one that will work for all characters is a jumping QK followed by a standing QP) and end it with his ultra move (forward, forward-down, down, down-back, back, QK) to perform your first ultra.
Next you will continue up the arcade ladder winning your next 9 matches before you reach Eyedol. Defeat Eyedol with Jago for the next achievement.
If you won all 11 of your matches without dying once you will also unlock the following achievement. Do not feel discouraged if you do not get it with your first character. You will have to play arcade mode 10 more times and you only have to go undefeated once.
ThriftyYou defeated Eyedol using only one credit

Next we need 40 wins total with Jago. You would have gotten at least 11 out of the way by playing arcade. You can either continue playing arcade mode until you have reached 40 total wins or you can plug in a second controller and defeat an idle player 2 over and over until you reach 40. For quick wins have both players, while selecting your respective characters, hold down and press MK. This will select the Sky stage as the chosen arena. Simply knock your opponent off the side to ignore their health bars and the matches will go by much faster.
During your 40 win boosting, either with Jago or anybody else, you will want to perform a humiliation on all 10 base characters in the game. To perform a humiliation you have to win both rounds of a match without loosing your first health bar. When your opponent is defeated and a voice overhead says "Danger, danger" input the following move to perform Jago's humiliation. (Forward, forward-down, down, down-back, back, MK) This will net you your first achievement for performing a humiliation, repeat this 9 more times against each of the 10 base characters in the game for another achievement as well...
ShamelessYou humiliated every character in the game

TJ COMBO
Next we move on to TJ Combo. Win the arcade ladder and defeat Eyedol making sure to perform at least one ultra combo along the way. To do this perform the lead in combo while holding forward then perform his ultra move which is to hold forward for a bit and then quickly hit back+FP.
Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
ORCHID
We start to repeat ourselves a lot for a while just win 11 matches in the arcade ladder, defeating Eyedol and making sure to perform at least one ultra in the process. For her ultra hold back while doing the lead in combo and then quickly hit forward+MP.
LasakenYou defeated Eyedol with Orchid

Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
CINDER
Bet you can't guess whats next? Defeat Eyedol in the arcade ladder and perform at least one ultra along the way. Cinders ultra is performed by doing the lead in combo and then inputting forward, forward, MP.
InfernoYou defeated Eyedol with Cinder

Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
SABREWULF
Select Sabrewulf, perform an ultra in the arcade ladder and defeat Eyedol at the end. Hold forward while performing his lead in combo and then quickly hit back+QK for his ultra.
HowlYou defeated Eyedol with Sabrewulf

Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times. During these 40 wins make sure to get at least 1 perfect win by not taking a single hit the entire match. This is obviously much easier with a second controller.
SPINAL
Now it is time to play as the skeleton Spinal. Same rules apply get one ultra and defeat Eyedol in the arcade ladder. Do the standard lead in combo then input the following quickly for Spinals ultra: back, back-down, down, down-forward, forward, FP.
Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times. Make sure to get at least one perfect win during these 40. This is obviously much easier with a second controller.
GLACIUS
With Glacius perform an ultra combo by using the lead in combo while holding back, then forward+FP. Defeat Eyedol in the arcade ladder, nuff' said.
Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
CHIEF THUNDER
Start an arcade ladder and defeat Eyedol at the end while making sure you perform at least one ultra combo along the way by holding back while doing the lead in combo then hit forward+QP.
SammamishYou defeated Eyedol with Chief Thunder

Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
FULGORE
For Fulgore you will have to perform at least one ultra combo during the arcade ladder and defeat Eyedol at the end. Perform the lead-in combo then his ultra move (forward, down, down-forward, QP)
Next get your 40 wins by either plugging in a second controller or repeating the arcade ladder a few times.
RIPTOR
For Riptor you will have to start an arcade ladder and perform at least one ultra combo during your climb to the top. His ultra is performed by holding back while doing the lead in combo then quickly hitting forward+QK.
Next continue the arcade ladder and defeat Eyedol.
RampageYou defeated Eyedol with Riptor

For the last time get 40 wins either in arcade ladder or with a second controller. Get one more perfect playing as Riptor as well during these 40 wins.
Monster MashYou got a Perfect in Arcade with Spinal, Riptor and Sabrewulf

EYEDOL
Now you will have to defeat Eyedol while playing as Eyedol. In order to play as Eyedol select Cinder for the arcade ladder in the 1.5 version of the game. During the vs. screen hold
and quickly press
,
,
,
,
,
. You should hear Eyedol say his name and at this point you can release
. Defeat Eyedol using Eyedol for the next achievement. Also make sure to win a normal match using Eyedol as well. Do not play through all 10 matches with another character and switch to Eyedol last second)
RANDOM
Now it's time to win a few more 2 player matches either online with a friend or against a second controller. By playing through the arcade ladder so many times you will have played in every single player stage in the game. There are five maps that must be played in multiplayer. You may have gotten them randomly by getting your many wins above, but if not input the following buttons while selecting your character to get the following stages...
Backstreet Stage =
+
Ice Stage =
+ 
Underground Lair =
+ 
Sky Stage =
+
(on both controllers)
Lastly there is one stage that you cannot select and have to just keep playing matches until it shows up. It is the ground version of Spinals stage. You will see the castle doors in the background. Once you win a match on all 10 normal character stages and BOTH Eyedol stages in the arcade ladder and won a match on these 5 stages you will earn the next achievement.
Next, if you do NOT yet have the achievement for spending 400 credits in arcade mode, now is the time to do so. You no doubt have had to restart the ladder quite a few times during your 11 runs of arcade mode so it shouldn't be too bad. You can now set the difficulty to the hardest setting and die, repeatedly. Hit continue every time you die and repeat this until you hit a total of 400 restarts. Note that the achievement app on your xbox tracks its progress at 1% for every 4 restarts.
If you have done everything I stated above the next achievement should have already unlocked, but this achievement is a little glitchy. If it has not yet unlocked then head to the games main menu and watch all the cut scenes in the game. Watch every victory animation and character intro / ending. If you have defeated Eyedol with every character in the game, gotten a supreme victory with every character and watched every cut scene in the game you will unlock your last achievement.