The problem I had was you are busy finding a good safe point to fight off the normal waves and you're taking them out (which takes 10 mins) and then like EIGHT rocket guys show up who turtle on the other side.
Now if the blast radius of the rockets wasn't HUGE, you could find a safe pillar or corner and just pop out and snipe/shoot one at a time until they are all dead. But KZ2 has the largest damage radius I've ever seen from rocket blasts and even if the rockets land nowhere near you, if several of them do at the same time you're dead.
I think the main problem actually came down to that aiming in KZ2 is more difficult than most FPS games because of the loose controls. This was fine for most of the game, but it made this section in the room very frustrating and near impossible. By the time I aligned my aim to shoot 1 rocket guy, I had 6 rockets flying in my face from the other guys.
I ended up winning by launching hand grenades from each side behind the debris and luckily take out 2-3 of the rocket guys on each side with one throw from each side of the room. Then I shot off the remaining one guy with an assault rifle.
Also it wasn't just the horde rush. I had no problems with the first checkpoint when you were on the ground with enemies everywhere. That was fun and didn't seem that much harder than the earlier stages. It was the upstairs rocket part + THE FINAL BOSS WHO WAS TERRIBLE.
Two things about the final boss sucked from a design standpoint:
1) If you run out of ammo (I did), KZ2 is the SLOWEST FPS for picking up a new weapon. OMG, you can't even run over a weapon and grab it while you are running. You have to run up to it, STOP, hold square, then start running while loading it. Unfortunately if the final boss has gone into teleport -> knife rush, teleport -> knife rush, cycle, when you STOP to hold square you die instantly from his knife rush.
2) If your 5.1 isn't very good (I was playing it on a different setup to play during the day in a dark room), it can be VERY DIFFICULT to hear where he is spawning. This led to many times where I was running around and ran backwards into him with a knife and instant death.
Overall I didn't think it was the whole game that was difficult. Going back through mission select I cleared most missions/areas with zero deaths. It's more that certain checkpoints had high difficulty spikes and that your weapons suck. A good example of this is the cruiser level. The first checkpoint when you get to the top of the elevator and there's one friendly behind a crate and like 2 waves of 6 guys come through a wall on the opposite end. this should have been easy. In my head it's as simple as throwing a grenade when they all rush out and then taking cover at the start and picking off whoever remains when they come at you. In reality this took me like 10 deaths and a lot of frustration RIGHT OFF THE BAT when the stage started. The grenades wouldn't kill them (I don't even understand how that works. I threw and HIT huge groups of 5-7 guys multiple times in the game with grenades and never got the multi-person grenade kill achievement because I guess it just didn't do enough damage to kill them @_@) and so they just keep charging and throwing lightning grenades back where we start so I'd have to move forward to avoid instant death from that and then there'd be like 3-4 guys shooting me at an angle that hit me even behind cover. Eventually a grenade took out enough of them that a bum rush was barely able to take out the rest, but a simple situation like that should not have been a choke point. After that I didn't die once for the rest of the entire level, so it was just that spike RIGHT AT THE START of the stage, which was frustrating.
Now going off that, things that made those parts harder than they were:
1) Your accuracy SUCKS in KZ2 with any non-sniper rifle. If you are standing 5-10 feet from a guy it takes an entire clip of unloading an assault rifle to kill him for certain because the aim spreads apart so fast while holding shoot down that probably only about 25% of the bullets are actually hitting the guy. Maybe Guerilla wanted to make a realistic gun porn game, but the realism of accuracy makes it less fun. Especially since these guys can take so much damage. You'll shoot them and think you've killed them and then they'll get back up and start shooting at you again. I ended up melee-ing a lot of people to death because at least when you keep smacking them you know they are DEAD. I was also forced to use melee a bunch because after several encounters where I bum rush a guy and shoot him from a foot away with all my bullets and run out and he IS STILL ALIVE AND STARTS SMACKING ME...yeah, not good. (Also want to mention as an example of how bad the accuracy/damage scaling is: They made an achievement out of killing 2 or 3 guys with a single clip of 320 shots of assault rifle ammo. I STILL HAVEN'T GOTTEN THAT even after trying purposely to do it standing 3-5 feet from a group of normal soldiers).
2) Aiming just felt slightly off. It wasn't a problem when you were playing the game rush style and always shooting from pretty close and jumping behind things to reload. But it made picking off guys from a distance without a sniper rifle a chore. Surprisingly the aim with the sniper rifle felt very precise, so maybe they purposely just screwed up all the other weapons.
3) Not really important but the shotgun is the worst weapon in any fps I've played. In a game like this where you can die in 1 second from close up enemy fire, how useful is a shotgun that doesn't even 100% 1 hit kill a guy a foot from your face? I shoot them in the head and they react and then recover and then kill me before I can get a 2nd shot off. @_@
4) Like I said earlier, you die too quickly; the damage scaling seems uneven (sometimes I'll take a rocket HEAD ON and survive, other times I die from the blast radius of one that misses), and there is no decent indicator of where you are taking damage from. The game needed a better indicator pointing in directions and after you die it should have had a kill cam so you at least know where to look out for next time.
Still, while it sounds like I'm being overly negative on the game it's because I thought everything else was AMAZING and one of the most visceral, intense FPS games I've ever played that was like nothing else out there. The level design was brilliant, with sprawling levels that took you to all kinds of varied locations (within the same level) and felt connected the whole time. The set pieces were really neat looking and usually varied in gameplay setups. The graphics were the best I've seen on a console. The great geometry + incredible use of motion blur and post-processing effects just blew me away. The animation was INCREDIBLE. The mech animation for the flying copter boss when it was swooping around all quickly is the defining graphical achievement for robots in games to date. Same with the animation when the exo-skeleton first gets up. The mecha design in general and overall art design was just through the roof in quality. The AI was also extremely impressive and the overwhelming odds + smarts of the enemy flanks brought the intensity and feeling that you were fighting your way through a real warzone. The gunplay felt extremely good and satisfying when you were taking down the enemies. The bosses outside the final one were great. Even the turret section in the sky gave a great Rebel Assault vibe with gorgeous visuals and decent turrent gameplay (was better than shooting asteroids >_<). The length was also just right for a SP campaign and the collectibles and achievements add a bit to round it out.
So yeah, I really like the game but I think it has annoying flaws that will keep me from replaying it in the future. I won't call them major or game-breaking flaws, but they are problems that hit the average skill fps gamer (I've never been too great at FPS games. I just play them on their normal difficulty levels and finish them all) and frustrate the hell out of them at times. The difficulty spikes were just very unbalanced and jumped far too high at times. Hopefully Guerrilla takes the feedback in hand and for their next game makes the challenge a bit more even across the board. If you want insanely difficult situations, then leave them for the higher difficulty modes.