Top 10 Hardest Parts in Doom Eternal

Mission: Sentinel Prime
I can't really put into words how much I struggled with this guy during my first Nightmare playthrough. Even though I remembered most of his attack pattern and frequently chainsawed the Fodder demons that spawn around you, this guy was still my biggest obstacle in the entire game. His hits also seem to randomly hit you, even if you're far away from whatever projectible is thrown at you, and staggering him with a good hit seems to happen randomly. I know this guy is a pushover for a lot of people, but not for me!

Mission: Cultist Base
After seeing the glorious Super Shotgun for the first time, waiting for you to rescue it, the floor underneath your feet drops down, leaving you in this claustrophobic square room. Things start off easy with a group of Fodder enemies. Then things escalate when a Whiplash and some Gargoyles are introduced, followed by an Arachnotron. You can use the Cueballs to take either of these two big targets out instantly, but it's going to make the final wave a lot more of a hassle. Two Mancubi drop down on either side of the arena while the walls catch aflame and the ground becomes electrified. If you don't deal with at least one of the Mancubi quick, this part will instantly kill you on Nightmare difficulty.

Mission: Arc Complex
About halfway through the mission, when trying to enter an elevator, the floor crumbles underneath your feet and you're dropped into a room with three Mancubi and a Pinky ready to eat you alive. Luckily, you get a teleporter and a Berserk power-up. You'll need both of them, because a Cyber-Mancubus, 4 Whiplashes and an Arachnotron will all appear in the arena at some point. I noticed the Arachnotron likes to camp right next to the Berserk power-up, in front of the teleporter, so keep an especially close eye on that monster. This is also the only part in the game where you can find the Berserk, so make good use of it!

Mission: Urdak
like the Gladiator, some people consider the Khan Maykr an absolute joke of a fight. Meanwhile, I died about 10x here on Nightmare before I could even get her staggered. The fight is pretty chaotic, since your main target shoots energy waves at you that will deal a lot of damage if you're hit. You also wanna keep an eye on the Maykr drones that are roaming the arena. During the end of the battle, the Khan Maykr will set a huge chunk of the arena on fire, forcing you to seek the high ground, where you become an easier target for her projectile attacks. Her "health" bar also regenerates if you stop damaging after about 5 seconds. The only way to get it down with one shot is to use the BFG. Luckily, the game gives you a single BFG shot. The lock-on burst is also very good against her, and if micro-missiles are fully mastered, she'll also be a bit easier.

Mission: Taras Nabad
The Arch-Vile is most dangerous in this mission, since you don't have your Crucible yet. This Arch-Vile in particular is very nasty. The room where it spawns in is tricky enough. It's dark, cramped, and has a fair amount of heavy hitters. Before the Arch-Vile appears, you have to deal with a Whiplash, Carcass, Hell Knight, Prowlers, and eventually a Baron of Hell. Good thing you get a BFG round!

You still don't have 2 blood punches to destroy cybermancubi so picking fights is the key, plus any surprise attach from a spiderhead, revenant and specter pinky can end your life quickly.
Look at youtube for tips on how to skip this fight.

Mission: Nekravol Part 2
Two Cyber-Mancubi, two Spectres, a Carcass, a Revenant and an Arachnotron. In an enclosed, not-too-big circular area. I think that's all I need to say about it

Not even 2 BFGs can save you unless they are perfectly timed.
Chainsaw and maneuvering are your only choices. There is not much cover so all enemies will gank you until you are no more.

Mission: Final Sin
I would say this is the most chaotic fight in the entire game. When you drop in, there's already a Tyrant, Mancubus, Arachnotron, and a Pinky. After the first Tyrant dies, two more spawn in, along with a whole load of other enemies. As if that wasn't bad enough, you also get a Buff totem to deal with while the Tyrants are probably still alive. You get a BFG round, but it doesn't make the room a whole lot easier.

Mission: Exultia
Although I personally only died here about 3x out of the countless times I played this mission, even on Nightmare difficulty, I can safely say this is the most stressful part of the mission. It starts with an Arachnotron, a Revenant and some Gargoyles, plus other Fodder enemies. You end up fighting two more Arachnotrons after the first one dies, a Hell Knight, a Cacodemon, and more Gargoyles. Bare in mind that you only have your shotgun, heavy cannon and plasma rifle at this point. Not exactly ideal for a fight of this scale.

Mission: Hell on Earth
Hell on Earth, being the very first mission, is pretty easy, even on higher difficulties. The Arachnotrons are your biggest worry here. Most of them are fairly easy to deal with, even if they're a lot tankier compared to your other foes in this introductory mission, and even some foes you won't encounter until later missions. The last Arachnotron you have to take down jumps up onto a platform where it stands taller than you, surrounded by Soldiers and Imps, with not a lot of room for you to move around. You're at your weakest at this point of the game in terms of stats, but what makes this fight especially difficult is the cliff between you and the big arachnid. You see, that cliff is preventing the Arachnotron from jumping to your platform, so you have to move in and get a bit closer than you might like. The turret is easy to deal with if you have precision bolt, but its grenades do a lot of damage. Not to mention, if you decide to jump over to the Arachnotron's side of the cliff, its melee attack can pretty much instantly kill you on Nightmare.

Mission: Exultia
Not difficult per se, just extremely annoying. This is after you obtain the plasma rifle and kill your first Revenant, along with another Hell Knight, Cacodemon, and group of Gargoyles. You have to retrieve a Sentinel Power Core, for which you have to travel through a lengthy bit of purple goo. The fact that you get slowed down to a snail's pace is bad enough, not to mention you can't even jump or dash while you're stuck in it, but to add insult to injury, you also have to deal with a total of 5 tentacles before you get to pick a new mod for your arsenal. If you know how to spot where they're going to pop out and slap you, this part is only mildly stressful. However, if you go through this part for the first time, you're bound to get a good scare here.
Pro tip: you might be tempted to use your shotgun to shoot the tentacles, but I'm telling you that Precision Bolt is the better method. So long as you point directly ahead of yourself, you'll only need one PB shot to kill a tentacle, even if you don't shoot it in the middle of its stem, like you have to do with the combat shotgun. Even better, if you activate Chrono Strike while in the purple goo, you have all the time in the world to take out the tentacle when it pops out.