Much like Roland in the original, Axton brings a deployable turret to the fight, but his skill trees let you develop him in a couple different ways. While Maya and Zer0 may be best built striking quickly then moving, Axton and Salvador are both built to be right on the front line, and their abilities help there.
Let’s take a look at Axton's skill trees - Guerilla, Gunpowder and Survival. After that, I'll provide some ideas on how you should build him.
Axton's Action Skill - Sabre Turret. Axton's turret has a full 360-degree rotation, and the ability to add rockets, multiple turrets, and other upgrades that will make your turret almost as good as a fifth party member. Not only will enemies also attack the turret, they'll be taking damage from it, which can give you a moment's pause to heal, reload or whatever else needs doing. The one major downside is that the turret has a pretty long cooldown, 48 seconds.
To make the turret even stronger, take a look at the skills from his three trees - Guerilla, Gunpowder and Survival.
Guerilla: This skill tree is based around making the turret a stronger damage dealer, dealing high-damage attacks continuously.
Sentry - Tier 1: Gives your turret +1 shot per burst and +2 seconds of duration per level.
Ready - Tier 1: Grants you +8% Reload Speed per level.
Laser Sight - Tier 2: Gives your turret +10% Accuracy boosts per level.
Willing - Tier 2: Gives you +15% Shield Recharge Rate and -12% Shield Recharge Delay per level.
Onslaught - Tier 3: Kill Skill, killing an enemy gives +6% Gun Damage and +12% Movement Speed for a short time, per level.
Able - Tier 3: Causing Damage to an enemy regenerates 0.4% of your Maximum Health per second, per level for 3 seconds.
Scorched Earth - Tier 3: Adds Multi-Rocket Pods to the turret, which fire 22 Rockets per Volley.
Grenadier - Tier 4: Adds +1 Grenade Capacity per level.
Crisis Management - Tier 5: Gives you +7% Gun Damage and +6% Melee Damage when your shields are depleted.
Double Up - Tier 6: Adds a second gun to the Turret, and both barrels fire Slag Bullets.
Gunpowder: Focuses mainly on Explosives, Weapon Damage, and Axton's Offensive Skills. Builds the Turret more as a distraction.
Impact - Tier 1: This is a straight +4% Gun Damage and +3% Melee Damage boost, per level.
Expertise - Tier 1: Gives you +14% Weapon Swap Speed and Aim Speed per level, as well as a +7% Movement Speed while aiming.
Overload - Tier 2: Gives you +10% Magazine Size with Assault Rifles.
Metal Storm - Tier 2: Kill Skill, killing an enemy gives you +12% Fire Rate and +15% Recoil Reduction, per level, for a short time.
Steady - Tier 3: Grants you a +8% Recoil Reduction, +5% Grenade Damage and +4% Rocket Launcher Damage boost, per level.
Battlefront - Tier 3: While your turret is deployed, you gain +6% to Gun, Melee and Grenade Damage.
Longbow Turret - Tier 3: Turns your turret into a Longbow Grenade, granting +10,000% Turret Deploy Range and +110% Turret Health.
Duty Calls - Tier 4: Helps you deal straight bullet damage with +5% Gun Damage and +3% Fire Rate with non-elemental guns.
Do or Die - Tier 4: This will let you throw Grenades while in Fight For Your Life, and adds +10% Grenade and Rocket Launcher Damage.
Ranger - Tier 5: Want to boost everything? This skill boosts your Gun Damage, Critical Hit Damage, Accuracy, Fire Rate, Magazine Size, Reload Speed and Maximum Health by 1% per level.
Nuke - Tier 6: When you deploy your Turret, it will set off a small Nuclear Blast.
Survival: This skill tree is all about boosting Axton's defensive stats, as well as keeping him alive, and using the turret frequently.
Healthy - Tier 1: Gives you +6% Maximum Health per level.
Preparation - Tier 1: Grants you +3% Shield Capacity per level, and you regenerate 0.4% of your Maximum Health when your shields are full.
Last Ditch Effort - Tier 2: Gives you +8% Gun Damage and +14% Movement Speed during Fight For Your Life, per level.
Pressure - Tier 2: Depending on how low your health is, you gain up to +14% Reload Speed and -12% Shield Recharge Delay per level.
Forbearance - Tier 3: Gives -8% Status Effect per level and +1% Maximum Health per level.
Quick Charge - Tier 3: Kill Skill, killing an enemy will regenerate 1% of your shield per second, per level.
Phalanx Shield - Tier 3: The Turret now projects a shield that tries to block incoming enemy fire, while allowing friendly fire to pass through.
Resourceful - Tier 4: Gives you +5% Action Skill cooldown per level.
Mag-Lock - Tier 4: Lets you deploy your turret on walls and ceilings.
Grit - Tier 5: Gives you a 4% chance, per level, to ignore damage that would otherwise kill you. Not only will you not take damage from the attack, you will also regain half of your Maximum Health.
Gemini - Tier 6: You can deploy two turrets at once.
Building A Better Commando
I really like the Ready, Willing, and Able skills from Guerilla, along with Scorched Earth to get you in to the fight. I think Scorched Earch combined with Longbow Turret and Nuke from Gunpowder makes your turret real offensive power. Toss in Mag-Lock from Survival and you've got a damn fine turret. You can probably skip Double Up and Gemini to take Nuke instead, but Gemini also could be really useful with a powered up turret.
For Axton himself, there are a few really nice skills in the Gunpowder tree. Impact, Overload, Battlefront, and Ranger all give you really useful skill boosts to maximize your damage potential. If you are more of a defensive character, I would grab Forbearance, Preparation, Phalanx Shield, and Grit instead. Keep in mind the make-up of your party and Axton can be a major asset to your team.
One last thing to note for all four characters - one of the Achievement/Trophies, Capped Out...For Now is to hit Level 50. The name of the Achievement makes me think that the level cap will end up being raised, so don't worry if you miss a few skills here and there.