Spacelords

I found this easier playing solo as the enemies scale with difficulty the more human players there are,

Luckily at this point the game is pretty dead and I managed to match make with bots into a game, I'd recommend playing a couple of missions first to see which ones you're comfortable with, for me it was, under adventure, 3rd adventure down, 2nd mission.

Took about 35 mins to complete, avoid elites to avoid unfortunate deaths, most enemies die relatively quickly but the eiltes can combo you to gain a kill so be vigilant.

Score is based on time taken, number of deaths, number of kills etc so I'd work on reducing your time by learning what you need to do on your mission of choice and just slaying what gets in your path.

Try not to fall into any map hazards, cliffs etc.
Took me 2 attempts after not playing for 2 years.

I used Alicia, shotgun and Wall jump comes in handy to get around and to put good damage down.

You can unlock training solo mode after completing a mission 5 times which is also worth mentioning should you keep finding actual players. *Though I'm not sure if you get rated on this mode as a disclaimer*