Go to battle options and press rt to switch to raids.
Look for an invasion raid - usually costs 40 fuel.
Now the challenge - to win ten of these!
In these raids you and three other players team up to defend a fuel pump against 3 waves. Each wave has two regular enemies and one leviathan. (these are player created once lvl 17 has been reached - these will likely get harder to beat as time goes on)
You each get one free respawn if you die or destroy yourselves. Respawns thereafter will either cost money or you can build respawns in the engineers factions (blue section) **the last update relocated some weapons into different groups so this could change with further updates***
My advice is to equip rockets, cannons or melee weapons like grinders if possible as well as strong guns.
I think the leviathans levels may depend on the group strength, a combo or the higher players structure strength but I am not sure.
A bit of luck and a good team helps but be careful of the pumps health as you only get one pump and its health lasts all 3 waves. Their is also a timer but you get extra time after each wave. Be mindful of the timer as it can be your enemy too!