Seth can solo the entire game he is in. I beg to differ. Majorly.
So I recently beat my Lunatic run. So I played on casual because I couldn't keep 90% of my units dieing in chapter 2 and 3. Also because I wanted to get female supports. Half way through the game I decided to say "Screw it!" and bought my maxed out limit breaker MU from my previous file and my maxed out limit breaker DLC Marth. It made Lunatic much easier except the last chapter.
I played the Endgame with only those two units and my MU and Chrom paired up with them. I barely managed to beat it. I went through 7 elixers and it took me like 20 minutes. It was hard. And I only beat it because I got lucky. I probably should've bought a couple more maxed out units. From this Lunatic+ has been unlocked for me (only casual mode though). Oh I tried the prologue and got ripped apart. Not touching that again for a long while.