Eventually the game will get whittled down to 15 players and it is at that point that you have officially entered the End-Game. Once you are inside the map circle the best strategy then to protect your back is to stay as close to the boundary wall as possible as it slowly continues to shrink in. Eventually as you are pushed together with the remaining players you will have to fight so make sure you have found a good position, whether it’s elevated land like a hill or being inside of a building.
Eventually when there are only 3 or 4 players left you will need to locate where they are and target them one by one. The best strategy at this point is to try and eliminate the biggest threat first.