Mercenary Squad is Chapter 6x of Fire Emblem: New Mystery of the Emblem. This chapter is reached if the player beats Chapter 6 in 20 turns or less on Normal Mode (each higher difficulty progressively adds 4 more turns to this limit). Alternatively, the player has to have Rickard recruited and alive in order to access this side Chapter.
If you wish to recruit both Caesar and Radd, this chapter can be quite difficult. If you have trained Marisha and she has about 8-10 speed (depending on the difficulty, she'll need a bit more speed to execute this strategy), class change her to a Mage and have her attack the Armor Knight directly above your party. Have Marth and another bulky unit who can kill the other Axe Fighter in one round kill the 2 Axe Fighters above Marisha, then send Linde on the the southern fort with a Nosferatu tome equipped (she WILL need this for this strategy to be executed). Send two fliers to dispatch the Hunters on turn 1, then remove all weapons from the final unit. As long as Marisha does not double the Axe Fighter that will attack her, and Linde does not kill Radd or Caesar (both of which are very unlikely), you may continue.
On turn 2, use one of the sword wielders or Marisha to dispatch of the Axe Fighter beside them, then send Linde to the spot where the Axe Fighter was. If you brought a sufficiently leveled Palla (around level 15), she'll make short work of Legion with the Silver Lance, other wise, attack Legion with as many units as you need to. Both Radd and Caesar will go for the unit with no weapons at this point, so there are no worries.
On turn 3, kill Legion if you haven't by this point, then seize the throne.
Alternatively, it's also possible to clear this chapter in 1 turn. Position a healer with the Rescue staff in the upper right location and a strong Paladin right below them. Kill the fighter above Radd, and send in that Paladin to defeat Legion in one round of combat (Palla, Sirius, and the Avatar are good choices for this). Reset if needed if you are relying on a one hit critical. Move the Healer as far as you can towards the top right, and have them Rescue Marth. Then proceed to seize.
2 Fighters Lv. 5 w/ Steel Axe and Vulnerary
2 Fighters Lv. 5 w/ Steel Axe
1 Knight Lv. 5 w/ Steel Lance and Vulnerary
2 Fighters Lv. 5 w/ Hand Axe
2 Archer Lv. 5 w/ Steel Bow
Caesar: Mercenary Lv. 7 w/ Steel Sword
Radd: Myrmidon Lv. 5 w/ Steel Sword
Boss: Legion: Berserker Lv. 1 w/ Hand Axe