One of the best merc to tackle this achievement is actually 'Wolf'.
As an allrounder, he is able to heal himself and with some trial and error and training, is capable of modding his equipment with the most difficult mods.
Stealth is paramount and all your weapons need to be silenced, except for the MG.
Get a sniper rifle as soon as possible and mod it with a suppressor.
As a backup weapon at the start, pick a shotgun or a similar close-range-high-dmg weapon.
Stay away from the refugee camp and let the endgame trigger by coming close to eagles nest. Be sure to get a stack of grenades or rocket launcher to get rid of grouped enemies in the final 3 fights/sectors.
By the end you should have:
2-3 Sniper rifles: PSG1/Dragunov since you can fire them twice, M82 for high dmg. Ensure that at least one sniper has a different caliber than your MG and ammo is plentiful
1 MG: ideally HK21; lie down and deploy it to deal with groups of 3+. Careful, it burns your ammo quickly and has the same caliber as your primary sniper rifle PSG1. Bipod is mandatory
1 Shotgun: AA12 for close combat
1 Assault rifle: ideally Aug with grenade launcher for grouped enemies. Serves as low AP silenced weapon and backup weapon if you're low on ammo on your sniper rifles. AP ammo is preferred after eagles nest
A major pain point is that you can barely handle explosives, can't craft the ammo you need and traps detonate more often. Be sure to save often.
Money is no problem, since you only have to pay one merc. Visit Fleatown often and buy books to level up as many stats as possible.
Traits:
Sharpshooter - simple, extra dmg
Killzone - extra attack during interrupt almost guarantees a kill
Reactive Fire - interrupt (two attacks thanks to Killzone) on enemy miss fire
Ambusher - higher chance of stealth kill
Beefed up - Face it, you're alone and every additional HP is needed
Distracting Shot - only way to deal with pin down and overwatch
Deadeye - greatly helps to convert every shot in oneshot-kills
Fast Runner - never let the enemy get too close. Free Move Range increase
Untraceable - easier maintaining Sneak mode
Equipment:
Light helmet with weave padding
Light camo armor (better for sneaking)
Flag leggins with weave padding
Increased movement due to light armor and Fast Runner trait. When facing isolated enemies, scout around them to ensure the kill doesn't alert others. If you're facing more than two, make sure to quickly move behind cover. Don't mind explosives and go for grenades right away.
Between fights, take the time to heal often and treat wounds if you're suffering from 3 or more.
Always repair your gear when it's below 70%. Sniper rifles tend to jam quickly below 60% and MGs break fast when overused.