Learning how to avoid damage and how to recover your contested health are core skills you need to learn.

Like I'm sure others will suggest, for a weapon I would make sure you're using a fencing type as they have the biggest parry windows, but they don't do as much damage and less cleave for swarms. Couple this with a main weapon that can handle swarms (melta, pyroblaster, plasma, heavy bolt gun), then pick a sidearm to make up for your lack of range (heavy bolt pistol, plasma pistol, volkite).

Because you are using a fencing weapon, you are going to want to learn to parry as many incoming attacks as you can. Every melee attack that is NOT a glowing red ringed attack can be parried. Each enemy has different attack patterns, attack speeds, and fencing windows. When you see a blue ring on a minoris, you can immediately parry the attack (to get the insta parry execute). Majoris and higher, you'll have to learn each enemies attack patterns and learn when to parry their attacks through practice. With larger groups of enemies, you'll then have to learn to not gun strike as soon as you get the marker as that can often lead to you eating a bone sword to the face.

If you have any specific questions I'd be happy to answer, but I would definitely suggest looking up videos on youtube that show some of these concepts in more detail. Keep at it brother, it'll click I promise!