Part 2 endgame

No not really. I think if geoffrey or elincia dies you get game over though. Word of advice, DON'T let nephenee die! Shes the best sentinal in the game

how is nehpeene the best

I believe Geoffrey or Elicia's death will cause a game over, but if the others die I don't know what might be left out.
I had the same issue with wanting to keep all of the characters intact. What I would recommend would be using Brom on the left to defend the stairs, and used Mordecai, Lethe, and Nephenee to defend the small entry-way past the door. Using Olivi Grass and Laguz Stones prove useful. I then used Callil's meteor to weaken strong enemies, or defeat ones with lower health. After that, I made Elincia inch closer to enemies on the middle-ground and used Leanne's gladr, used her to kill enemies with Bowguns or Crossbows (with Amiti), then retreated her back to the higher ground. I had Marcia deal with any other arrow-wielders that Elincia did not get and then had her go defend near the stairs using her Javelin to finish off enemies. When the middle area was cleared of enemies, I brought Callil down to deal with enemies on the lower ground with Elfire and Elincia to attack from the back of the rebel army. Once Geoffrey's team comes along, just hold out for a few more turns. Focus on a hit-and-run strategy to weaken the boss, and then I used Elincia to defeat Luvdeck. That was my strategy.