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:
name: Kanesada
found: Stage 21
how to get it: Simply buy in the events menu (about 30 gems)
name: Ichimonji
found: Stage 27
how to get it: Given as reward for completing "Rescue the Princess 2"
name: Munechika
found: Stage 36
how to get it: Recieved after completing a special mission called "Kagemusha"
name: Murasame
found: Stage 41
how to get it: At events menu "Gift from a peasant" (30 gems)
name: Yasutsuna
found: Stage 45
how to get it: Given as reward during "Treasure Hunt" mission
name: Sukesane
found: Stage 51
how to get it: Given as reward during "Swordsmen's Challenge" mission
name: Koshino
found:Stage 71 and above
How to get it: Received after completing the special "horseman" mission on Stage 71
name: Osamitsu
found:Stage 81 and above
How to get it: bought for 30 gems on Stage 81
name: Odenta
found: Stage 86
How to get it: Complete the "Dream Team" Mission
name: Yasatsuna
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.