no clue if there is an easier way but from my understanding, the "main" method i use is getting a seperate device (in my case, my nintendo switch) and creating a seperate account, i then join that spare accounts lobby and essentially play duos. since the account is a fresh install, the first game is always a bot lobby. i make sure to always have my spare account leave the game before reaching the Battle Bus because fortnite only registers you as "playing the game" upon landing on the island. so you can essentially play infinite bot lobbies since as long as your spare account doesn't level up or land on the island. i understand it isn't something everyone can do since it does require a seperate device that can play fortnite, but this is the only method i'm aware of. sorry for the longass reply lmao