There's numerous things I could tell you, but it'd be easier to remember if I just list the most important things to keep in mind that could literally make or break a game (not just your performance in a game):
Communication- If you have a mic, inform your team the moment you see a scrake or fleshpound (or both) and where it is. If you don't have a mic, type in chat sc or fp and where they are if you can type fast enough. Much more on this in the other points.
Choosing the right team- If your team has no Medic, either be the Medic or leave and join another game (at least for suicidal+, hard could also very easily be a disaster if the players aren't competent at healing themselves). Having a Berserker to act as a tank for scrakes and fleshpounds is also ideal, perhaps necessary.
Also, only play with teams who are appropriately leveled. You could probably do alright with one player who is very low level (as long as it isn't the Medic or Berserker), but 2+ is most often going to be a major problem. As a general rule, the levels should be: Hard 5+, Suicidal 15+, HoE 20+ (ideally 25)
Stay AS FAR AWAY AS POSSIBLE from Hans- Hans does significantly more damage on Hard+ and can take you out in a second with his melee attacks. Also, if you see your teammates too close to Hans, TELL THEM! (Unless they're a Berserker of course). Even if you don't have a mic, it's more important for you to type it out in chat than to do damage as a dead teammate will do far less damage. And if you see teammates standing over Hans' grenades, again TELL THEM! (assuming you have a mic). And stay in view of the medic! Which leads me to the next point.
-Always stay in view of your medic (or at least have the option to get in view of your medic very quickly when needed without the chance of being overrun). This is especially important against Hans. And if you see teammates fighting Hans or scrakes/fps out of view of the medic, it doesn't hurt to remind them to stay in view (especially if they're taking significant damage and the Medic has to constantly struggle to get over to them).
If you don't use effective communication, you'll notice a huge difference between how your team performs. I've found playing without my mic to be incredibly difficult. Any time I want to play games without it, rarely does it turn out well (assuming other players don't have/or aren't using one).
For Suicidal+-
Conserve ammo- Use your pistol whenever possible for single low threat ZEDs. You'll quickly run out of ammo otherwise. If you're Commando/Support, headshots are crucial. And as Commando, you'll want to use single shots every chance you can (when things aren't too crazy).
Get good with hip fire aim- You'll get overrun very frequently depending on the team, so having decent hip fire aim is incredibly important so you can move quickly while firing and conserving ammo (not missing too many shots).
This could really apply to any difficulty, but is extremely important on Suicidal+. Try to get your team to fight Hans in the most open part of the map- It's best to fight him in the most open area of the map for three reason:
1. Hans can more easily stay in view of everyone, ensuring that everyone can put out a steady stream of damage on him at all times.
2. It's easier for everyone to stay at a distance from Hans so they aren't ***** slapped to death in a second.
3. This is probably the most important one, people are far less likely to get out of view of the medic(s).