There process becomes a lot faster once you beat the labyrinth and unlock the exploration of it, specifically, it's nightmare mode. If you can find a consistent way to beat multi enemy encounters and boss encounters without losing much spirit, specially the imp den event, you can advance somewhere around 20, 21 levels per trip? You mileage will vary, since the extra xp from nightmare mode doesn't quite match up to the rate at which your xp requirements scale, so it slows down after level 100, and also because if you choose not to use only events and instead use encounters, these all have a chance to be single enemy encounters, however small, which will slow down your rates a bit.
The good news is that you can also use the labyrinth to grind at some encounter based romances as well, and a lot of them do lead to encounters, meaning you're not even slowing down the xp grind that much for it.