Fire Emblem: The Sacred Stones/Chapter 8: It's a Trap!

Seize the throne
For this battle, send Eirika, Seth, Colm, and maybe a healer to meet up with Ephraim. Send everyone else up to the door, making sure that Artur is one of them. Don't open it yet though. Have Artur kill the Shaman on the other side of the wall.

Defeat the units that come at Ephraim from the way he came, and send Seth and Colm up the path to the Armor guarding the door with the chests. If you want to level Colm, be careful, as the archers will pick him off quickly. Keep Colm two steps behind Seth so he doesn't get hit. Seth can easily take care of all the units along the path, and the armor at the door as well. Once all the units are defeated, send Colm into the room to unlock the door and chests. Now, destroy the cracked wall and have the units with Eirika and Ephraim meet up with Seth and Colm. Make sure to have Ephraim and Eirika talk to each other, since it unlocks a song in the Sound Room.

Now, open the door you left your other units by, and send in a character with an axe. Don't use Ross unless he's upgraded. Kill the armors inside and head to meet up with the rest of the group, send Colm to the room with the chest, and get the chest. Now you're free to attack the boss and end the level.

Boss: Tirado
Tirado can either be a piece of cake or a real annoyance. With having more defense and a Silver Lance, he can pose as a real threat to just about you're whole team. It would be easier to have Seth take a hit from Tirado's javelin so that he's not as strong. Afterwards, send in someone with an armorslayer, rapier, or reginleif for extra damage. If he didn't die, then rescue, drop, heal, attack. Rinse and repeat until he's defeated.

After this chapter, you're given the choice of whether to follow Eirika or Ephraim. The one you choose becomes your main Lord for the next eight chapters, then the other one rejoins you. Everybody you've recruited so far will go with the Lord you chose. There is a save point after the chapter and before the cutscene, so you could save, watch the cutscene, choose one Lord, save again under a different slot, reload the first save, skip through the cutscene, choose the other Lord, and save again in the original slot. Then you'll have one save for each Lord and can play through both paths.