>>713610674
Technically almost all the FE games can be solo'd with a single character if you know how to break it.
I'm no expert but off the top of my head PoR might be the easiest to do so since it's an easy game in general and you can baby them with bonus EXP. Getting the full trio of shota characters would take a while though, so you'd have to deal with Rolf for early game.