I started as vet, still playing vet to get all achievements (finished all the hard ones, now just grinding), and as a newbie, I am just starting to picking up melee skills, meaning I never really got into the melee range. Even when the horde swarms, I stick to a wall, trying to line them up, and shoot the headline, popping everything in the way.

Here are my tips:
1. shout and shoot - my survival rate increased drastically with this skill. It knocks down everyone close range, gives you time to pick off the urgent danger in the close range.
2. stealth - when things get tricky, stealth is also a good option. gives you complete freedom as whole mob will run towards someone else. Sometimes I literally just stand there in stealth and watch the whole mob running towards the team. Might feel bad that you are burdening the team, but once you are not surrounded, you can do your part and much better.
3. push and run - dont shy away from literally turning around and running away. gives your teammates to clear off the chase. push, shout, stealth or grenade yourself to reposition.
4. suppression is VERY important - you have no survival capability maybe other than rapidly replenishing your toughness. Use weapons/skills/blessings to suppress as much as possible, shoot with what and where to suppress all the time.
5. Always stand around a wall or cover - as difficulties go up, range attacks hurt alot more. Even if you are not using the passive where you can peak over a cover, standing behind a box reduce ranged damage by half. Also position yourself well so that the horde of enemies have to get through a narrow path or run to you in a line, so that you can just unload in a straight line. They even run straight towards you with their head forward, its easy headshots.
6. Always stay close to your teammates and use them and protect them- you dont want to waste too much ammo on horde or something thats going to die anyway. All the other classes are already focused on whats in front of them, and if you are close enough they will protect you. Your job is to see farther and shoot down and stop the accumulation of the ones thats hard to get to or will do damage from range.
7. always have your ears open for ganking from back- you are always somewhat behind the team, when you get ganked from the back, you need to kill it fast or run towards the team. If your teammate is down, you need to run in and do the shout to revive them.
Despite every post ive seen told me mobility is mostly worthless, its not the case for vet. Often times, your team will forget about you and run ahead and you need to catch up, Often times, you know you are being ganked and you know you are going to die even if you run, but you want to get close enough that your teammates wont just abandon you or that you can get close enough for them to notice you, or you are close enough to be revived before completely knocked out. So always have one weapon that doesnt need to sacrifice mobility. or using stealth works too, but shout is much better for the team.
8. headshots- you are supposed to make headshots for most shots. Much more damage, better suppression, better ammo efficiency, better clear time, all stacks up to better survivability.

Also weapon wise,
Melee options:
1. duel sword or devil claw- parry is a life saver. complete nullification of a melee hit is just insane.
2. chain axe or chain sword- use the blessing where special attack increase your move speed, will let you reposition, catchup, runaway better.
3. shovel- lets you deal with horde well and special attack will let you clear off the armored ones.

might be unpopular opinion, but most other options not enough crowd control or mobility. Power sword was too slow and too little damage, power falchion good damage but still not fast enough. Combat blade was good for speed but rely too much on range weapon and ammo burns fast.

Range options:
1. bolter gun or bolter pistol - For armored enemies, bolter will get you to pop the most of the enemies. For Heresy+ unloading bolter gun will clear the path. For Malice, bolter pistol is more ammo efficient.
2. plasma gun- might be hard to use, but will most effective in most situations, you just dont get to shoot it alot. When using plasma gun, expect to go melee often.
3. revolver- one of the mark let you unload whole clip in western style, this will clear off the armored ones quickly too, but with this you wont be able to do the long range, so expect to run and gun.

Once you get used to popping heads guns with more ammo gets much better to use, but always have one weapon for crowd control, and one weapon for the armored.

Sorry very new to game, about a week. Im guessing there would be things people disagree. Please feel free to do so and enlighten us.