To me, there are only two people that matter on Lunatic: Chrom, and Robin simply because them dying causes a game over while everyone else doesn't.
Basically, from the prologue to chapter 4, I only allowed myself to gain experience while everyone else (except Chrom) served as my iron wall. I did something in the prologue I've seen called the "water trick." For some reason, Chrom and Robin are capable of running in the water.
So, I paired myself with Lissa and took out the mages that were across the bridge. Once they were out, I put Chrom and Frederick in the water while I took out everyone I can. As for the 78 turns. I had to take out one person before the boss but myself and Frederick were low on health. I used up all my vulneraries so I had Frederick go around in circles forcing the enemy to chase him while I got Lissa to heal me. I healed everyone, took out the remaining enemies and was victorious! I was at level 5 at the end so I was able handle the rest of the chapters. Though chapter 2 was more me sending everyone to their doom while I did all the work.
I had a similar situation in Inigo's paralogue though that was more me going in unprepared
again and I left with myself, Chrom, Donnel, and Sully as the survivors.