Summarize the following:
If you are invaded while fighting an enemy you have a little time to defeat it before your opponent can to you, depending on where they are spawned. Don’t get outnumbered or let someone catch you in a bad position.  If you are near a boss door, you can pass through it to end an invasion, then teleport home with a homeward bone. If you want to avoid PVP completely, you can play offline or play through the game hollowed. However you will also lose the opportunity to co-op with other players. Attack with a full combo, but leave yourself stamina to roll. You might be able to trick your opponent into thinking there’s an opening where there isn’t. This can be especially effective with a heavy weapon and light armor. If you can ever get behind an opponent, take the opportunity for a critical strike. It only takes a light attack and does not leave you vulnerable like a parry. Fighting near trees can give you cover from spells. Fighting near cliffs may cause an opponent to inadvertently fall while rolling. Lava is dangerous to everyone - but exciting. Spear users should find narrow spaces to fight. Enemy swords will bounce off walls. Being human is a double-edged sword, allowing for invasions and co-op. Take advantage and bring help to fend off invading players. In Dark Souls there are no rules. All strategies are fair game, especially when you are the invader. This can mean attacking your opponent while they fight someone else or running away to heal if you’re given an opening.

Summary:
Know when to run and when to fight. Bait out attacks. Fish for backstabs. Use the environment to your advantage. Summon for help. Do not play by the rules.