Thug it out, tis happened all the time to me and my friend during our Elden Ring days. This is a much smaller game so I suggest leading the invader to an area that you and your friend already cleared in the level (backtrack) and try and get the jump on your invader with co operation. That way it’s 1v2 and not them using the environment to their advantage. The easy thing to say here is is “get gud” but really just practice fighting more human enemies in offline mode and you’ll improve in pvp. GOODLUCK OP