Outriders will have a lot of RNG and that shows in the demo where we can get Legendary drops. Here is how to farm them quickly but keep in mind you're not guaranteed the loot.
To farm the highest level of Legendaries in Outriders demo, you will need to progress the story all the way up until Shira sends you, Jakub, the doctor and the sniper to The First City mission, which is the objective that will end the demo. You will be able to pick up a side quest from her at that point, called Terra Infirma.
It will have you trying to save Lieutenant McCain, who is trapped under rubble. Just go through the quest until you reach the final fight against a bunch of enemies that include a Captain. It is this guy who has a chance to drop Legendary weapons so you will just need to rush past everyone and kill him alone. If he doesn't drop the Legendary, let the regular enemies kill you and repeat the encounter.
If you are not sure which battle is the final one in Terra Infirma, check out the video below, which also showcases the fastest way to dispose of the captain, averaging a kill every 30 seconds.
One thing to keep in mind is that you can get Legendary drops as soon as reaching World Tier 3 but to maximise your chances, it has to be World Tier 5. Each time you die to enemies, World Tier XP bar will drop a little but will equalise with experience gained from kill the Captain once it reaches around 10 per cent.
If you slip up and get killed before downing the Captain a few times, you risk losing more experience but just make sure to kill a few adds on the next run and the XP should be back to around 10 per cent again.
From that moment on, it's all just RNG and repeating. Good luck!