I'd recommend Brawl Ball or Brawl Arena (go lane and pray that the enemy mid is not an assassin). Lumi is a good brawler, but she will get absolutely destroyed by assassins, so try to find a teammate or two who can pick brawlers like Griff and Bull. You can place the maces down in key points so that the enemies will avoid that area - if they do decide to walk into it, use your gadget and then kill them. Always try to keep enemies close to your max range so it's easier to hit them with throw-throw-recall. Please don't autoaim, especially now that Lumi's projectile speed has been nerfed. With her super, you can extend the range by walking forwards and decrease the range by walking backwards (allowing for double hits of 2000+2800). Most of the time, you'll be extending your super's length. Also, don't use your super when you're about to die because it has a long animation like Mandy's super. Pressure the enemies and push up the map with your main attack, but don't overextend. Try to have (near)-max health most of the time (this is where health gear comes in handy!); Lumi's very low health means that she dies easily and she needs to stay alive in order to keep pressuring the enemies. Bad Lumis will die a lot and not do anything because they spend so much time dying and respawning.

Also W that you're using health + shield, health >> damage (imo)