This guide should help you have the best gladiators in the empire and millions to burn at the end of the first year. The key is selecting the best of the best prospects (best of A+ scouted gladiators).
My ludus setup for best results is included.

The only tough part is the beginning, before you get your first million. To get there, here is my favourite setup / start :

When creating a new game select 'Gladiator' to have extra skill points to assign.

Get loan(s) ideally with interest rates 3 percent and less. And immediately purchase most lucrative business (or businesses, depending on the amount of reasonable loans).

Hire scouts with grading at least 470 and networking above 250 at least (you should be looking to build scouting team with grading 490 and above ASAP). Before you fill up your barracks, have as many scouts as possible.
If starting loans and/or scouts are terrible, personally I restart the game.
To start with, you will likely select whatever the best prospect is available (Should be A- overall and above).

You can see the order in which I build upgrades in the screenshot.
For office perks, I go in this order : Avid Recruiter, Scholar, opportunist. And then linguists to increase damage. I dont find anything to increase income or morale necessary. Though if you go for the highest economy difficulty, that might be a different story (have not tried that yet).

For research, I go in this order : Calisthenis (AP increase), Nutrition (HP Increase), Advanced Weapons Technique (damage increase) and then I dont have a priority, whatever I currently need.

Note!
I don't think I would be able to get such unbeatable gladiators if it weren't for the 'Avid recruiter' office perk maxxed out, and selecting your ludus boss to be a gladiator himself!
So if you are hoping to find a legend like on the screenshot below (all attributes 200 after training), I think you have to do these.

The maximum of skill points that can be assigned to a gladiator is 2000.
(10 attributes, 200 max per attribute)
When scouting, your goal should of course be to select the prospects who already have the highest number of skill points assigned (that is on lvl1, before training).
To determine which those are (as the skill points can be spread variously), you can use my openoffice spreadsheet or do it just by rough guessing.

a) Spreadsheet[we.tl]
Download[we.tl]

There you input the number of A+, A, A- etc ratings per prospect and get an overall rating. Anything 90 and above is an awesome prospect who can become an unbeatable legend easy.
The spreadsheet is self-explanatory and you can adjust the values and ratings as you wish, to make it more accurate for your purposes. I tested this on more than 40 A+ prospects and adjusted it as best as possible based on my experience.
Please note that you will rarely find 90 and above prospect who is 27 or younger. (but there is still plenty of time to dominate, create a legend and make millions)

If you reach a 100 and above, you may actually have a gladiator who can fill his attributes to the max (see my gladiator 'God of War' in the screenshot, you can also see in spreadsheet with what attribute ratings I hired him). As you can imagine he was unbeatable (well, all 90s and up can be).

b) Guessing
Basically, if the gladiator has at least 6 of 10 attributes in the top 3 rating (A+,A, A-), take him.
At the start I would take of course less than that, but eventually you should have your barracks filled with legends (or potential legends), and anything less than the best of the best simply won't do..

You can just calculate (add up) the number of skill points of hired gladiator on lvl1 for your future reference and comparison.

I usually train the hire all the way up to 40 (unless Im in 1st year) so I can see what the gladiator is really made of before fighting or releasing him. In the spreadsheet you will also find the amount of skill points assignable after the gladiator is trained up to lvl 40 for my gladiators (kept them there for your reference)
After the first year, I only select those prospects with rating around 90 and up.

Next, how do I assign the skills and expertise points to make the gladiator unbeatable?
(you can already guess from the screens I think)

How exactly you want your man to fight is up to you, but I'm providing rough guide for both skills and expertise, with which the gladiator can beat 3 opponents at 40 or so level himself (and make you a killing from a bet in a single fight).

Assign skills in a way that the equipped gladiator has the following abilities:

Action Points (kill em before they swing a sword!) - at least 20

Hit Points (withstand criticals) - at least 4000, if you want to fight regularly against more opponents with high level, I would recommend 5000 (max vitality and stamina)

Initiative - will be high with A+ prospects anyway, I don't look at this one at all

Hit Chance (make those action points count) - at least 75 percent, when you get to higher level opponents, they will heal with block, dodge and counter. If you have hit chance below 75, against best opponents you might damage him less than he heals and will lose. You may make it with less and very high critical, but against defensive opponents lvl 40 (and more of them in a single fight), you will need as high a hit chance as possible.

Damage - with top A+ prospects selected (based on my guide), its unlikely you would have to look at it too much, unless the prospect is Thraex (low weapon damage) and strength was his lowest attribute. Strentgh I usually leave lower anyway, as one of the random opportunities is a legend willing to train your gladiator in strength to max (not sure if he does other attributes as well)

Critical (insta-kill and heal!) - my favourite, killing opponents with one swing. Fully equipped (charms too) I aim to have 50 percent or more. This is key for fighting 1v3. You need to lower the number of your opponents as soon as possible. Usually I kill 2 with my first turn (1 if all are lvl 40). You will also need it for healing.

Critical Damage - again, should be very high anyway, never found the need to look after this.

Mitigation - ignore, based on armor

Block (heal!) - if you want to fight 1v3 and make tons of money, this, as well as dodge and counter are absolutely vital. Fully equipped I aim for 50 percent and up (45 percent should be bare minimum. With expertise assigned to healing, this is what will take you to the top and make you unbeatable.

Dodge (heal!) - same as block

Counter-attack (heal! and kill!) - same as block

Special cooldown - ignore, I rarely even use it with my gladiators, unless its inspiration (+3AP)




Offensive:
Fill Calculating nature and opportunist (5+5). Increases Critical chance and heals you when achieving critical - a must for 1v3.

Defensive:
Fill Lethal Counter, Walled Fortress, Inspired Agility and Sturdy organs (5+5+5+5) - again a must for 1v3. Heals during defense and sturdy organs are needed to reduce critical damage - some opponents will stun you with sand (no defence, all hits) and you will simply have to withstand a turn.

At lvl 40, and spending the points as described, you should have 9 expertise points left. If I really want to have the gladiator as 1v3 specialist, I put them into Hard Target (which should put you close to 8000 HP with top armor) and makes you withstand those stunned turns, couple of lucky criticals or turns where three high level opponents hammer at you.

Essentially, you aim to create a gladiator that heals more damage than receives and needs just one or two hits to kill anyone.

If you have read the previous parts of the guide and played the game before, you can probably guess how to make easy millions now.

You fight 1v2, 1v3 as much as possible, max bets on yourself and particularly during festival (purse multiplier).
Even if you have a legend, the odds are still good in 1v3 regardless of opponents.
If you followed the prospect selection and skill/expertise assignment, you should not lose even when simulating all the time. One festival (with a bit of luck in scheduling) can sort you out for years financially.
Also take advantage of injured, fatigued or sick opponents, the attributes are significantly lowered (ie they fight like c*ap), but the odds on the bets don't seem to be.

For tough 1v3 matches, I highly recommend using sand!
You will find many high level opponents with skill and expertise assignment similar to what I described (though not as specialized) and you will need the sand to get over their defense and healing. With the described gladiators, using sand, just one or two hits should down that mofo.
Should you find a 'God of War' like I did (see screen), it's not necessary, but still the safer option.

I only lost once this way and through a fluke... 3 high level opponents (1 was down already), sanded/stunned and received 3 criticals in row. (Opponent training, matching and equipment set to normal difficulty)

Do NOT share the fight purse!
Even if the gladiator is rapacious, it's always better/cheaper to donate via profile screen and raise his morale that way. This is particularly important at the beginning where you need every cent. In a year, you won't know what to do with the money you made from bets and you might as well share 100 percent of the fight purse (it doesn't make sense, but you easily could).

That's it, If I forgot something I will add it.
My all time list in my current game:

If you are wondering why some of my best gladiators don't appear there, it's because some became old before reaching the all-time numbers (Attributes seem to decrease from 31, when they age up) and I released them. Others haven't gotten to all-time yet, but will (Sharpie, Reggae Ambassador) no question:

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