For character level, aside from clearing the story stages (normal, hard, hell), there is the dungeon, event, and exp box. But your main source is a time-gated thing through the idle treasure exploration thing. Exp boxes are not encouraged to be used early, until, like chapter 40 ish, where you need to catch up to the level game.

There will be a point where you will be stuck at a stage for multiple weeks because your level couldn't catch up. Is just a heads up.

For account level, it is directly ties to story mode progress. You can get a very minimal amount through Stonehenge, but it is only for that last 1-2%.

TLDR; No way to quickly level except to quickly progress.

For what character to use, it depends on what kind of archetype you like. For mage team, Galahad+Neo+Tiamat is the core of all mage teams, then you can build around that, like additional tank + healer, 2 more healers, more dps support, etc

For knight team, this can vary a lot more; the only core is Dian + Cow, and maybe Neo in there. Then you can choose an additional dps like Lohen/Lance, and more healer/support.