Spartacus Legends

While this is a full guide to getting this achievement, my primary focus is on the strategies and button combos to use for each fighting style.

I've added my own details on how to properly bail to the dashboard - to both avoid losing your fighter and for the Gold Exploit. For those who are interested in a list of fighter Perks upcoming Multi-XP Events, please see CounterInsurgnt's solution.
Also, props to all the guides on TA that were posted before this; between solutions and comments, both helped me to get through this.

Primus Fights are basically story fights you must encounter along the way through the game. Legend fights (or Boss Battles as they are listed as under Help in-game) are also considered to be Primus Fights/Battles.
There are 6 Districts that each have a set of Primus Fights that must be completed for this achievement to be obtained. When selecting a District, there will be a Percentage on the left “Primus Fights Completed __%”. What is required of you, is to get all the __%’s to equal 100%. As you complete the Primus Fights, they will vanish and allow you to move on to the next fight. Upon completing all Primus Fights and earning the achievement, they will all be made available once again. So when the achievement reads “…for the First Time” this is what it is referring to.
There are 6 Primus Fights in each district. In total, there are 36 Primus Fights needed for the achievement.

The Districts & Legends from Each:
Extremus – Oenomaus (Sword & Shield)
Mercatus – Alcolytus (Dual Daggers)
Domus Nobilium – Masonius (Spear & Shield)
Oscan Capua – Crixus (2-Handed Sword)
Insulae – Ixion (2-Handed Hammer)
Spectacula – Spartacus (Dual Swords)
*Note: There is no Legend for the style 2-Handed Trident

Gladiator Tiers/Levels:
White: 420 Silver, 1 Perk
Green: 2020 Silver, 2 Perks
Blue: 5780 Silver, 3 Perks <-- Recommended to complete this achievement
Purple: ???? Silver, 8 Gold (until FL 21), 4 Perks
Gold (Legend): 8-12 Gold, 5 pre-determined Perks
Gold (Custom): ???? Silver, 13 Gold, 5 Perks (unlocks at FL21)
Red: ???? Silver, ??? Gold, 7 Perks (unlocks around FL31)

Gladiator Slots:
Make sure to expand on your available gladiator slots when possible. While you shouldn’t need Legendary Gladiators to complete the game, I did end up getting two of them which helped me progress through the game. My numbers may be off, but new slots approximately cost in the following amounts: 1000 Silver, 3000 Silver, 6000 Silver, 17 Gold, 33 Gold, and 111 Gold. If the gold exploit does not work for you, you still want to at least get the first slot that costs Gold.

District Guide:
For those of you towards the end of the game, you can skip over most of this, for those that are new to the game, or just need some random tips, feel free to read through the following. It is a strategy/guide/tips on what may help you progress through the game.

Key Fights:

General Team Guide

The Fighting Styles and the Best Methods to crush your foes with them:
Critical Attacks are key to this game. Technically the best combos that the short-range fighters have that can be spammed over and over involves a 3-hit combo, with the 3rd hit being a critical hit (or the chance to perform a critical hit anyways). The critical is important because it often staggers your opponent leaving you time to get in and start the next combo. Otherwise, you will leave yourself open at the end of your combo. Additionally, it is often that the critical attack will do damage regardless of whether or not your opponent is blocking. It may be chip damage, but damage is still damage. The weapons with long reach may not benefit as well from using combos, but instead, single attacks that are critical hits work wonders for them.

When it comes to purchasing weapons, you want to have Excellent Parrying, since we are constantly on the offensive, being able to avoid an opposing attack as we are attacking is great to have. Rolling, can be very useful based on who you are fighting, or what style you are using. While not as important as being able to Parry, being able to Roll is quite helpful. Blocking can be useful based on your fighter's style, the Sword & Shield makes the most use of it. Dodging... is shit, don't bother.

Dual Swords:
The two florentine (dual weapon) fighting styles are both very good at being spam-tastic against the AI.
With Dual Swords, use: Dwn+X,X,X (Repeat until your opponent is dead)
You also have an option of Up+Y when you are at a distance, and this covers ground quite quickly. However, this does have a long cooldown time that may leave you open to be hit.
Alternatively, if you are against foes with long reach, Rolling down (towards the screen) when you are at a distance (at the beginning and middle of the fight) works well. You may still get hit once, but it will help you avoid a full combo and get you in close to follow up immediately with Dwn+X,X,X.

Dual Daggers:
Same strategy as the Dual Swords, you want to use Dwn+X,X,X relentlessly. As with the Dual-Swords style, this combo is great because mid-way through the combo, you are actually ducking down as you attack, which will usually cause your opponent to miss you as you close in for an attack.

Sword & Shield:
This style has the same Dwn+X,X,X combo as the two above, however, it is much slower. Actually knowing and getting the timing down for when to block and when to attack goes far with this style.

Spear & Shield:
Until you know what you are doing, the Spear & Shield is by far the hardest style to use.
Up+Y works wonders (it is a red-crit/high critical hit chance), if you bloody/stagger your opponent with it, follow up with Fwd+Y. Make sure to make use of rolling with this class. It does not necessarily only need to be backwards, it can be up and down as well.
Up+Y is when you start with the spear low and then perform a jab straight to the head. It is one of very few attacks that is good at keeping your opponent at a distance. It should have more which is unfortunate, but at least it has this one. This is your key to defeating the Primus fight vs. the Dual Dagger opponent.
That is the only strategy needed for victory, I have no others moves I would suggest.
Feel free to try other combos, but ask yourself, "Why am I swinging a spear around like a bo-staff?" Also, if a fighter closed the distance between you and the tip of your spear in real life, you had a dagger on hand for close range attacks.
The Spear & Shield has a 4-hit combo that I DON'T recommend. Fwd+X,X,X is great unless your opponent moves...

2-Handed Sword:
I prefer to use Neutral+X,X,Y, as it ends with a crit, Up+X,X,X ends with a spin, but isn't really fast enough to allow for good recovery, nor is it good for dodging. Bck+Y,A,A is a a really good attack for when your opponent is far/approaching; you perform a spin attack followed by two punches that are fast and will take you from one edge of the screen to the other. It's a good punishment for taunting opponents. There may be a better combo, but I haven't found it yet.
The 2H-Sword also has a charge attack (A+X) that you can hold down and release for more damage. It is useful against weapons with short range, but long range weapons may still be able to attack you while you attempt to charge the attack up.
Beware of a 2-Handed sword when wielding Dual Anything, if you get parried it could mean your death. 2H-Sword users seem to recover quickly in response to Dwn+X,X,X, and can get damage in where other styles cannot.

2-Handed Trident:
The trident can lead you to many Perfectus victories, but you must know how to use it. It is one of the best styles in this game despite not being very spam-tastic.
You really only need to know one move when it comes to using a Trident:
Dwn+Y is a mid-range attack that aims down at your opponents feet. Make sure you start at a good distance away from your opponent and just repeat the move over and over as you step closer to your opponent. Often, if you've hit your opponent twice in a row with the move, you have gotten too close (against a dual-sword user such as Spartacus), and you should back-roll away. You should also back-roll away in general if you feel like you are within reach of your opponents weapon. If your opponent Parries or Dodges, immediately back-roll away. If you are going to be hit by a combo, make sure you do your best so that only the last hit makes contact by keeping your distance and rolling. Don't forget that your opponent covers ground, advancing as they complete a combo.
Your hardest matchup will likely be a Spear & Shield user. The same strategy above applies, however, Back-Rolls may be substituted with Upward or Downward Rolls depending on the situation.
This style also has a charge attack (A+X), I have no idea what it does, because I'm satisfied with Dwn+Y.
Warning: Up+Y may seem great, but when you are performing it right next to your foe and it just goes past him over and over, you may end up regretting it.
AI Tridents seem to be good at destroying 2-Handed sword users, so beware if you are one of them. If you somehow find the Net that a Retiarius (trident gladiator) is supposed to carry (first, PM me...seriously), it is very good at slowing your opponent while you stab them to death...

2-Handed Hammer:
A decent combo for the hammer is the same Neutral+X,X,Y combo as the 2H-Sword, it is a quick Jab-Jab-Swing(Crit) combo.
However, the BEST strategy with a Hammer user, is the charge attack:
A+X is a giant swing of the hammer that eats your opponents health. Make sure that you start far away from your opponent when starting the charged up attack. Release the attack as your opponent makes their way towards you, as they come into range. This attack is long-range despite the hammer itself being a mid-ranged weapon. ROLLING is key when using the A+X attack, after the attack is completed roll away. The only time you shouldn't is if your opponent is on the ground and you can get another quick hit in. This is the strategy needed to defeat Baal.
Aside from Ixion, most of the hammer users are a joke and can easily be destroyed by a Dual-Weapon fighter.
WARNING: The A+X Charge attack does use up some of your defense meter when it is performed, be aware of how much you have in your meter, because you don't want to be left in a position where you cannot roll or defend yourself.

Cestus?:
....is not in the game? If you have the option to not equip a weapon and just attack with your fist, why not add spiked boxing gloves? Either way, while it doesn't do much damage and although you have basically no protection, the fastest and longest combo belongs to the Unarmed category. I wouldn't recommend using it other than to get the achievement for using all the fighting styles.

"Shout his praise to the ****ing Heavens!"


EDIT:
How to Avoid Losing, and My 2 Cents on the Gold Exploit: