I understand this is Dark Souls and it's supposed to be hellish and hard. But the invaders are armed for PvP and usually appear while the hapless PvEer is already fighting mobs. It's not a duel, it's slaughter. In the unlikely event one comes through while you are in a cleared area you have a chance of winning because you have estrus and they don't. Their advantage is having fought dozens of duels. But the basic invasion rule is just to make the game harder and more unpleasant, because it's Dark Souls. This is why I waited until 2019 to play, most of the ***hats who enjoy slaughtering people who are engaged with mobs have moved on.