One of the most ubiquitous complaints about the first Division was that enemies were far too resistant to damage. It was all too common that normal human enemies in the endgame could ignore the effects of getting shot in the face repeatedly. It would be one thing if the game had a fantasy or sci-fi setting but the game takes place in modern times. As far as we know, no amount of modern medicine can allow people to eat their body weight in bullets. Fortunately, developer Massive made some massive strides to making enemy health and damage more believable in The Division 2.
For the most part, The Division 2 plays like a generic third person shooter prior to reaching the endgame. You get shredded if you're outside of cover, but you can drop enemies just as quickly. Enemies get a little beefier once you reach the endgame, but it's not much of an issue. Eventually though, you're probably going to reach World Tier 4, and this is where things get weird. After getting shot by smaller bullets for 30 hours, enemies have developed an immunity to larger and larger bullets. Red and purple health enemies are tolerable, but yellow health enemies turn into walking tanks.
Simply take a look at the final stage of the Roosevelt Island Stronghold on Challenging with a full group to see how enemies no longer care about bullets. Players die after getting shot several times, but basic enemies can (and this is not an exaggeration) shrug off 300 machine gun bullets that are being applied directly to the face from no more than 10 feet away. To make things worse, the AI is smart enough to abuse their health pools, and they will just march at your spawn area like a homeless Terminator. The best part is that audio cues for flanking enemies seemingly aren't playing because they're not flanking you so much as walking directly at you. Crowd control effects, like being set on fire, barely stop them.
To be fair, Strongholds on Challenging are supposed to be the most difficult levels in The Division 2. However, it does bring up an important question: what makes a game difficult? Virtually every looter shooter answers this question by boosting the health and damage of enemies to astronomical proportions. This leads to the common complaint that enemies turn into damage sponges. It's difficult to be sure, but it's not a good feeling of difficulty. You can't really learn anything from such an experience. Well, unless "shoot them and don't die" is considered a novel lesson. In such a situation, you're not fighting the enemy, you're fighting the game. The fact that some gear perks exist only to boost player damage to endgame enemies instead of doing something interesting is disconcerting.
Now that's not to say that no other game increases the damage and health of enemies to create a feeling of difficulty. No other kind of game genre does it to such an extent though. Look at Dark Souls. It's widely regarded as one of the most difficult games available. Boss fights tend to be these huge ordeals, but no one really complains about them. Why? Because every attack of every boss has some kind of cue. You can tell when a boss is about to insta-kill you or transition to whatever phase because of how they act. The Ornstein and Smough fight literally has a small cutscene showing a boss powering up after you kill their partner. Learn their cues, and you can dodge roll your way to victory.
For another example, look at the Mile High Club level on Veteran from Call of Duty 4: Modern Warfare. Extremely difficult, but you (theoretically) become a better player through the process of beating it. You need to remember where enemies spawn and where to throw flashbangs, how to aim better to maximize your efficiency, and so on and so forth. The same can be said of Overwatch. To climb the Competitive ladder, you need to learn ability effects and cooldowns, positioning, Hero counters and team comps, and so much more. Rainbow 6 Siege? Corners and angles. All of these lessons can then be translated to other games. A good feeling of difficulty gives players an opportunity to learn. A bad feeling of difficulty leaves players feeling helpless.
All that being said, The Division 2 is in a good spot overall. It has an amazing amount of activities and loot, the levels are varied enough, and enemies are generally quite fun to play against. The problem only lies in World Tier 4 where enemies are playing a completely different game. For players, the rules are that you stay in cover or you die. Meanwhile the AI is doing its best to emulate the Heavy from Team Fortress 2. Even so, World Tier 4 Strongholds on Challenging are beatable, but with Tier 5 right around the corner, things are unlikely to get any easier from here on out. The real test will be if Tier 5 will be the kind of difficulty that is fun, or if it ends up being an arms race between developers and players to see who can inflate their stats higher.