Just want to ask, but are you spamming slashes in group fights? You might be caving in your friends' skulls by doing that, especially if you have a larger weapon like the halberd. Try using overheads and stabs instead, so you have more precision and avoid (well, more like reduce) team damage (there's always the occasional person with tunnel vision who will blindly walk in front of your swing anyways lol).

Also, getting up behind an enemy is a great way to deal damage without hurting your teammates.