I'll see what I can contribute here...
1. Obective Play: Killing enemies is 'never' as important as completing or preventing the objective regardless of if you're the Antagonist or the Raider. People often forget this and believe that the more kills they get the better they are. There are various 'choke' points in all missions that should be paid special care to because they can easily be taken advantage of by a learned Antagonist.
2. So Many Weapons: Having different weapons are nice but people largely seem to misunderstand that they are 'different' or situational and not exactly better per say. You do not need to play as much Antagonist as people seem to believe to access building blueprints. Also, you can lose and wont lose 'any' Stigma 'until' you reach a threshold of 20. Lastly, as far as I can tell you 'keep' your highest rank achieved and 'that' is what the game uses to decide if you can unlock weapons but someone would need to confirm this.
3. Get Tactical Babae: Try playing the game less like a meat and potato shooter because it is not and you're going to get killed... a lot that way. You are overall and regardless of character fairly squishy and that time between getting shot and healing is when you really should avoid further combat unless you like that crawling animation. People often forget their 'abilities' are most useful when helping the team and/or progressing the objective. For example, is your team mate getting shot and you're playing as Lycus then throw that shield up and absorb those shots for them as this works wonders even on the Kuzmann boss fight.
4. Stick to it: Right now, the biggest complaint is matchmaking and that likely wont change as much as anyone would like. The best solution to this is simply form a persistent group of four people and you should be able to play 'any' stage you like straight away. Be forewarned, your group is only as good as your weakest link (protecting them mitigates this) especially since an Antagonist will quickly single that person out for rapid-fire kills. Regardless, you might want to always give some thought to protecting Harec, Shae and often Alicia as they tend to be prime targets for easy kills.
This is probably not the best but a lot of this was stuff I noticed a lot of people just did not seem to be picking up on...