This is some strategic loadout ideas for the Mercenary. It is based largely on opinion, and these suggestions may or may not fit your play style or player level.
These are the weapons the player starts with when beginning a new save.
Primary: MK 48 - An average machine gun. Stats are balanced across the board. Though there are much better MGs than this, such as the Minigun and the Cerberus.
Secondary: Kriss Kard - An above average pistol. The Kriss Kard tends to excel in a lot of departments over other pistols.
Description: Due to the MK 48's mediocre accuracy, crouch to increase your aim. It would be wise to upgrade from this loadout as soon as possible.
Medium Range Loadout
Primary: Any MG with a high DPS.
Secondary: Katana - For close quarters, can reflect projectiles.
Attachment: Red Dot.
Skill: Ammo Feed - Negates the need for reloads, especially useful since machine guns have slow reloads.
Killstreak: Surge/Bloodthirst - Allows you to out-DPS any enemy that isn't wielding a Sniper Rifle or Shotgun.
Description: This set works well at medium and especially close ranges. However, it is prone to being outranged by Sniper Rifles and higher-leveled AI. It also loses to long-ranged Shotguns due to their much higher DPS.
Long Range Loadout
Primary: Any MG or Rocket Launcher with a range of at least 30 ft.
Secondary: A pistol of your preference/Katana.
Attachment: Red Dot/Target Locater.
Skill: No Sweat - Allows you to abuse explosives at close range, also great against opposing explosives.
Killstreak: Surge/Bloodthirst - Allows you to KO Mercenaries and Juggernauts much easier/Fully restores your HP very easily.
Armor: Military Jacket - Explosives don't have that much ammo reserve.
Strategy: This is basically for those who hate dying that much. Shoot rockets from afar. Don't worry about the aiming that much as the weak homing capabilities of Stinger isn't *that* weak. However, you'd still need to pick up ammo often.
Explosive spam loadout
Secondary: Throwing Knife - Not a must but just in case.
Skill: No Sweat - So you don't blow yourself up.
Killstreak: Death Wizard - So awesome with explosives. If only the Commando had higher damage it would've been the best weapon for this loadout.
Strategy: So the strategy here is to "kill forever, shoot forever". Shoot rockets until you get the kill streak, use it and spam rockets as you collect supplies(i.e. Health, Armor, Ammo).
Primary: Minigun - High RoF and decent damage.
Secondary: Katana - For melee uses.
Attachment: Red Dot - To increase the accuracy slightly.
Skill: Scavenger - So you won't have to worry about running out of ammo.
Killstreak: Bloodthirst - The high DPS will heal you quickly.
Strategy: Try to get within range of your minigun. If your opponent is targeting you, then jump in an semi-circle over him and shoot at him while doing that. He will try aim at you but will do it slowly. He should be dead by the time you land.
Primary: M32 - Fastest ROF Grenade Launcher
Secondary: Go ham
Attachment: Go ham
Skill: No Sweat - Prevent Self Damage
Killstreak: Bloodthirst - Heal
Armor: Kevlar Vest or anything with good armor
Strategy: The goal of this class is to rush someone while shooting with your grenade launcher. While the M32 isn't the highest damaging Grenade Launcher, it has the highest ROF making this class almost a run 'n gun. If need be, you can still get an angle on your target and spam those grenades. No Sweat allows you to rush past an enemy with the grenades you just left for them and Bloodthirst helps you in a pinch.
Primary: M60 - Pretty balanced stats all around.
Secondary: DEagle - Best in-class damage and range
Attachment: Red Dot - Better accuracy on the M60
Skill: Ammo Feed - Never worry about the tedious reload time of the LMG
Killstreak: Bloodthirst - Stay alive while mowing down the bots
Armor: Go Ham. Choose something that works in tandem with your health as you'll be using it to regenerate lost hp.
Strategy: You're basically turning yourself into an Engineer that doesn't need to build robots but instead relies on your vampire killstreak. The M60 w/ a Red Dot is basically giving you a Minigun trading ROF for better range and accuracy. Ammo Feed helps with the long reload time and basically gives you a free Incendiary rounds. Use the DEagle to finish people off and Bloodthirst to stay alive.
Primary: Cerberus - Excellent ammo supply and damage, along with surprisingly high accuracy for machine guns.
Secondary: Go Ham. Choose something that can back you up effectively, most likely a good pistol such as the Kriss Kard.
Skill: Scavenger - For near-infinite ammo, which works amazingly with minigun weapons like the Cerberus.
Killstreak: Bloodthirst - It can refill your health quickly, thanks to the high DPS of the Cerberus.
Armor: Titanium Plate - Insane protection while in action. Alternatively, Kevlar for good armor without sacrificing your health.
Strategy: Compared to the Minigun loadout above, this loadout is primarily mid-ranged. Compared to the Minigun, the Cerberus overall has a worse DPS thanks to its lower rate of fire. So this should be a slightly longer-ranged alternative to the Minigun loadout, thanks to the Cerberus' innately higher accuracy.
Fast Leveling/Kills Loadout
Primary: Cerberus or Minigun- these are the best machine guns, and having a machine gun is beneficial against mercenaries cause it can't be resisted like explosives.
Secondary: You won't really use them much anyway cause machine guns have huge ammo pools so use whatever you want.
Attachment: Red Dot- for extra accuracy.
Skill: No Sweat- explosive resistance.
Killstreak: Bloodthirst- amazing killstreak with machine guns, you could get a lot of kills without dying when it's active.
Armor: Blast Plate- explosive resistance.
Strategy: This loadout takes advantage of stacking the 30% explosive resistance of No Sweat with that of the Blast Plate to get 60% explosive resistance. What you do is to make custom death matches (of 50 kills) against mercenaries only. A lot of the mercenaries would use explosives, and with the 60% resistance you'd have a huge advantage over them. You could tank more than 2 direct hits from a rocket launcher and a lot of grenade hits while your machine gun will kill them fast so you could easily dominate these matches. Beware of machine gun users though.
This is probably the most efficient Mercenary loadout for getting a lot of kills fast, and you can quickly level to 50 with it and get money to buy new weapons from the slot machine.