Games : How does EXP work?

I can't beat level 120 (Mega Glalie) so I probably need to grind my team a little more.

So I'm now wondering about the leveling mechanics. They are quite vague, you can't tell how much EXP a pokémon gained during a match, if there are certain factors to how much they gain...

For example:

- Do my Pokemon receive EXP if they're just in my party or do they only get it from combos?

- Should I focus on higher levels or do they level up the same for no matter which level I choose?

- Is expert mode a good way to grind?

The only thing I know is that the Pokemon who does most damage in a match gets a crown icon with boosted EXP. But I don't know how much.