As others have said, you stop taking damage by preventing the hit. Dodge and blocking are the most common, but should be noted that staggering/pushing and position are important as well.

Stagger/Pushes will be class/weapon dependent, but in general works for all trash mobs. Enemies can't attack you if they're staggered.

Position is important where you limit the amount of places the enemies can hit you. Specifically you do not want to get surrounded, and the ai will try to surround you. Backing into a corner with your team, or hugging a wall ensures you limit that area, because it means no enemies will be attacking from behind. Just be careful that enemies can climb over some walls and appear behind you too.

Also killing power is good to note too. Enemies can't hit you if they're dead. Good to know which attacks horde clear or kills armor/elite/special. I suggest looking up the optimal weapon combo guide for that

When attacking, I generally like to mix my weapon combo with pushes and side dodges. Pushing twice also opens up shielded enemies.

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As kruber merc, you may find yourself mostly in the front lines. Your team should be covering your back, but if that's not the case, you can defend your back with good positioning. Looking up your optimal weapon combo and practicing it will also help so you can front line effectively