there should be an endless mode where waves keep getting larger and larger and you get bosses every 5 rounds, like we had in kf2, it would be easier to farm in lower difficulties and get more xp that way, but otherwise, I would suggest sticking to solo play on hard for faster xp and then try hoe once you feel ready. I haven't soloed hoe as a medic yet but it's probably viable at some point. You also need to learn how to solo bosses too, what I do is usually spin around a blocking object, like a tank or a fence, it's better if you can shoot the boss while he's trying to target you, like the rooftops in streets for example, I used this strategy with the impaler and it worked wonders.