Mercenary achievements guide
There are 2 achievements in Mercenary mode
Mercenary Mode: Unification
Claim victory in a Unification Battle in Mercenary Mode(SW2XL) and save your game.
To do this, keep fighting for 1 same lord. For example, if you pick a mission,
under the mission name there will be "Date VS Shimazu", you will be fighting for Date clan in this mission. After a few missions for Date, you will get to choose a event to ally yourself to Date clan. Pick that, then pick playing missions for Date clan, eventually you will get an option to do Unification battle, win it and save. Should happen around level 50.
Mercenary Mode: Greatest
Become the greatest in the land in Mercenary Mode(SW2XL) and save your game.
There are 10 precious swords, you need to obtain them all, and beat level 100.
They come in no particular order.
Four of them can be bought though merchant offer through events (not shop), you will get event like "Special offer from Merchant", pay 30 gems to get sword.
Other six can be obtained though missions, the missions are:
Escort Princess - part 2
Defeat the fake No
Defeat sword master
Rob the rich
One man riding
You need to win those missions to get the sword. The last sword mission comes at lv 99.
The fastest way to get both achievements is to join a clan, once you can pick the unification battle mission, do it, but save to another slot to get achievement. Then load previous save before the unification battle to continue on to lv100.
I will play it again to make sure I didn't miss anything.