Dandrum Fortress is Chapter 11 of Fire Emblem: Thracia 776.
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
This chapter has an annoying reliance on luck that unfortunately cannot be avoided. Before you go in, you should bring both Lifis and Lara with some lockpicks and maybe have Leif hold a few door keys.
The first thing to note is that, in order to reach the gaiden chapter, you will have to finish it in less than 30 turns and Fred has to be alive. With this in mind, try to take out the Soldiers and Archers near the entrance as quickly as possible. They are low leveled and generally easy, so try to give the EXP to any weaker units you are trying to train if you can. The archers also have Killer Bows if you want to capture them.
The easiest way to deal with entering the fortress is to place a powerful attacker like Orsin (make sure it's NOT Leif) in the corridor to kill all of the wandering Armor Knights. Once they are all dead, position Lifis, Lara, and the unit you used to kill the knights in the box outside of Fred's range. He won't move, but will still attack if anyone gets within two spaces of him. On the next turn, make sure no one is in the corridor and have Leif enter the box. Immediately after he enters, two ballistae will show up and three locked doors will appear in each corridor. Fred will become non-hostile and you can now pass by him. Immediately open as many doors as you can with the units you already had inside the box and try not to leave any frail ones (like Lara) within range of the ballistae.
At this point the chapter is mostly about luck. Hopefully your units will survive the turn and can keep moving forward. From here you should send your fighting unit in to finish off the enemies, send one of the thieves to pick up the torch staff in the chest, and get to the boss as quickly as possible. Be wary of the bishop who can attack you from behind the wall. Kempf will flee after being engaged in combat once, so don't be too intimidated by his stats. Once he's gone and you've opened the chest if you wanted to, seize the throne immediately.
The reason for all the rushing is that, once you leave the box, the ballistae will start targeting Fred. He can take three shots before dying and will only use a vulnerary when he can only take one more. If you get unlucky, he can get hit once on one turn, then twice on the next, preventing him from ever healing at all. Because he's technically not on your team you can't use healing staves on him and he will only sometimes decide to move so that he's not in range of both at the same time. Any two turns are enough to kill him and he can very easily run out of vulneraries, so whether or not he lives or dies is entirely up to luck; the most you can do is finish the chapter fast and give him less time to get unlucky.
One Final Thing you could do, if you have a Safy/Nanna with A Staves, you could Warp skip this chapter. Because the boss runs away right after you attack him, there is no risk putting a High Level Unit/Promoted unit to attack Kempf, so if you have the Warp staff and a A staves user, you could potentially do this, if you don't care about getting Chapter 11x.
- Just as in previous chapter stealing bolting sword from Fred makes Fred respawn with new one in the next chapter. If you stole bolting sword in Noel Canyon chapter and this one you can have total of three bolting swords when Fred joins you in the next gaiden Chapter 11x.