Becoming first mercenary is easier than you think.
First off, the earliest you'll be able to this trophy is level 47. That's the lowest level you'll have at even being able tp challenge the top mercenary, who is level 50 himself.
I'd suggest looking up their weakness' and strengths before trying to fight one. Some have no obvious weakness, so the fight is avoiding their strength. Some have pets tp help them too, ignore the Mercenary and go after the pet, particularly if it is a big Cat, as they climb on your back and make you easy for the mercenary to pick you off.
Don't hunt mercenaries lower down the chain than yourself, their rewards are normally next to useless, but on the flip side they are helpful to raise the "Killing Mercenaries" challenge which unlocks some very nice engravings toward killing the top dog.
Make sure you use this trophy to go after the "Bloodsport" trophy, wait til an mercenary with an obvious weakness is in the arena and spam the hell out of the weakness to gp towards this trophy and "Bloodsport".
Also, it's worth noting, if you get your backside handed to you by an mercenary, escape his view and wait for your health to naturally regenerate (theirs don't as long as you're in proximety to them), gives you the upper hand when re-engaging them.
You can chain together mercenary kills easily by using the bounty system to your advantage, I climb up four places in one fight by killing a leader in the middle of the port to raise my bounty to maximum. This also unlocked another trophy in the process. Make sure to upgrade ALL your chosen gear to your level if you plan to do this, especially if you're playing on the harder difficulties. This will be as most of the mercenaries to come at you will be higher leveled than yourself.
Also, Make sure you exploit the Tombs toward thia trophy, extra skill points mean you can have stronger skills for the fights. Tier three skills go along way to helping against the mercenaries tier 3 and above.
Three skills that come in handy are "Arrow master", "Critical Assassination" and "Hero strike". Two are in the Assassination skill line and one (obviously) in the archery skill line.
Make sure your healing skill is also at rank 3/3 when you get to teir 2 yourself, as theae mercenaries really do lile evading more often and their attacks hit hardest. Learn these mercenaries skills and their actions before activating skills, this way you'll know how to evade damage. Their unparryable skills also hit harder than normal fighting, so make sure to evade those, perfect evade is better when doing this as it slows down time to let you exploit that mercenaries weakness.
save the most powerful mercenary in a group of them til last. Pick them off in level order, otherwise you'll become desynchronised, the larger/higher leveled enemoes tend to back off in large groups and let lower levels attack you until you're nearly dead and they get the final blow.