Eh, Lunatic's fine. For the first bit, you just have to rely on Frederick to tank, and feed your Avatar lots of EXP by abusing the Veteran Skill (1.5X EXP gain when Paired Up). Your Avatar will be way over-leveled, and then when you can get Second Seals, you can re-class into Sorceror. From there, you can quite literally beat the game without effort just with Chrom and your Avatar, by giving your Avatar Nosferatu and having them Paired Up with Chrom. And that's not even including Spotpass Bonus Box stuff, or DLC grinding.
Lunatic+ will require luck though. Because while the Lunatic+ Skills are powerful, what really makes it luck-based is that they're randomly given to enemies, so they won't always have the same set of Skills with each playthrough. This leads to situations where in the first portion of the game, you might as well just restart so that you don't have to deal with un-winnable circumstances.