In order to get this, you must get all of the swords then do the Greatest in the Land on stage 99 (or 100, not sure which).
Here are the names, stages found, and how they are obtained for the swords:
found: Stage 21
how to get it: Simply buy in the events menu (about 30 gems)
found: Stage 27
how to get it: Given as reward for completing "Rescue the Princess 2"
found: Stage 36
how to get it: Recieved after completing a special mission called "Kagemusha"
found: Stage 41
how to get it: At events menu "Gift from a peasant" (30 gems)
found: Stage 45
how to get it: Given as reward during "Treasure Hunt" mission
found: Stage 51
how to get it: Given as reward during "Swordsmen's Challenge" mission
found:Stage 71 and above
How to get it: Received after completing the special "horseman" mission on Stage 71
found:Stage 81 and above
How to get it: bought for 30 gems on Stage 81
found: Stage 86
How to get it: Complete the "Dream Team" Mission
Found: Stage 99
How to get it: Reward from the shogun (40 gems)
**I do not take credit for this guide, it came from here: http://www.neoseeker.com/forums/36359/t1139503-mercenary-mod...
This place gives you LOADS of help on Mercenary Mode.
Thanks to Johnus Maximus for the last 2.