Fire Emblem: Shadow Dragon/Chapter 25

Chosen by Fate ''The long war had been fought, Akaneia's many lands freed, and now the League had cornered their arch-enemy within his castle. They broke into four groups and stormed the four gates of Doluna Keep- unaware that this was exactly what Medeus was expecting. Bravely, these warriors fate had chosen steeled themselves for one last struggle.''

Preparation
This battle has a lot of preparation going on. First of all, if you usually pick twelve or fewer units, you have the advantage here. The game recommends splitting into four parties, but you can choose to only deploy troops in three of the four regions. This gives you the advantage of not being spread out too much.

Be sure to have units in every region except the north-west. Don't bring any units that will drag you down, only the best are needed. Bring the strongest weapons: Falchion, Gravidus, Mericdus, etc., give a Door Key to Marth and use Lena's hammerne to repair Falchion, Gravidus, Mericdus, etc.

Begin
Gotoh is in the area next to Marth.

First turn, use the Door Key to open the door. Converge your units toward the enemies situated in the middle area. There are a lot of them, and it will most likely take the best weapons to prevail without risking a unit too much.

If done correctly, most enemy units, except one (in the extreme lower-left), should have been wiped out from the left side.

The doors will close in unnecessary places after all your units move past it. Note that enemy reinforcements will still spawn here, so you will need ranged units to take care of them.

Send Wolf or any extremely strong unit to the north. Use Parthia on the Manaketes and any weapon on the heroes.

Send a whole bunch of units up, and do the same as last map. The reinforcements are long. I mean long. They can last as long as 50-60 turns!

However long the turns are, it still means that you will need to cover the staircases near the boss' right (a Dracoknight with Javelin spawns there) and to the left (ballistae that can pick off units trying to fight with Medeus spawn too).

To the left, a ballista will continually spawn and they will harass your units who are trying to kill Medeus. Make sure to send an archer to defeat them.

Four units spawn in the bottom also, they are annoying, especially the Manakete I kept some of my strongest units, the paladins, behind to take care of them.

Another reinforcement is to the west, in the room. You can ignore it if you want, just keep in mind it has a ranged weapon and will attack.

It may take a while, but you will prevail eventually. Fortify is your best friend here. When this is all over, all that is left is Medeus. Bring in your best units and get them fully healed.

You can have four units at max crowding around him attacking: three ranged and one physical. Watch your units HP and keep in mind that the throne will heal him.

I recommend sending in your units that have the regalia: Parthia, Mercurius, and Gradivus. They plus Marth's final attack (of Falchion) should be enough to kill him. Keep in mind that the best way to attack is to let him attack you first, so as to avoid his counter-strike.

Also note that Medeus has a HORRIBLE attack rate, he attacked twice and missed both times. Of course, I got lucky with a critical, cutting his HP in half.

He isn't hard compared to other bosses, his hit rate is only 50% and you can also heal with Lena. Also, Tiki is a good choice to deal good damage.

Quick strategy
If you have tiki, aum staff, and a warp user you don't need to worry about the rest. Warp Tiki next to Medeus and fight him. Revive Tiki with Aum staff. Warp Tiki again and defeat Medeus.

Alternately: if you have Tiki, Falchion, and two warp users, warp Tiki near Medeus and fight him, then warp Marth near Medeus and defeat him.