This way is very simple. You only need:
1-3 very good healers (Pent, Sarah, and Priscilla are my best choices)
All the stat improving items from earlier in the game. (Recommended)
Around 3-5 units that can take heavy damage and dish out heavy damage. (Harken, Vaida, Oswin, Marcus, and Dorcas/Bartre are good choises.)
2 Strong horseback riders/fliers.
Nills is highly recommended
Before the battle give all of the items to Athos. (If you have them) Now he is a lot better.
During the battle... Well I just have a good way to go at the end of this level... So just kill all of the people that come out of the side room. Before you kill the last cloned boss to come out, send all of your units to the top of the stage by the big door. Except for some strong units at the bottom. Leave him/her at the bottom. Your 2 strong Calvary/fliers right next to the door on each side; put Athos in the middle. When you kill the last boss, 3 generals should appear from the bottom and the big door should unlock. Send your Calvary/fliers to kill the druids but make sure they aren't in range of Nergal. Now get everyone out of the way except for, you guessed it, Athos. Have Athos use Aurellia -or that light magic attack he had at the beginning of the level- and have the healers use physic on him. Repeat until he is dead. Whew. You aren't finished yet though. Now you have to kill a...Dragon. It's okay though have 3 strong defensive people hold the lords if they are weak (this guide was kind of made for if they were weak) and send Athos and the healers to the dragon. Have Athos use Forblaze on the dragon, it should attack twice. Heal him at least 1 space with physic away because the dragon attacks from 3 spaces away like a longbow, and if you were there, you'd probably be out one healer. Repeat until the dragon is dead. Now you have finished the game, try playing on Hector's Mode.

In the beginning of the game when you have Lyn as your lord it's ok if your people die because you get them later in the game when you start Eliwood's quest on chapter 11

The Mine Glitch. This glitch enables you to take control
Of the enemy units for one turn. Upon using the mine, and having the
Enemy trigger it, quickly soft reset the game and start again. Now
You can control the enemy units! This also works in fog of war maps,
But you have to click around some to find the enemy units you wish to
Move. This can also be used to have the enemy drop their weapons, or
Even have them merch them to Merlinus! This is how the Uber Spear is
Obtained.

Now that you know how to use it you can use it to your advantage.
1. Get free weapons to Merlinus
2. Get the Uber Spear
3. Get recruitable enemy character faster by moving them closer to your ally that recruits them and get the weapons that couldn't reach Merlinus