It's not that darkzone is harder than reasonable, it's that all previous levels are way too easy (to the point i suspect many players give up on the campaign, as it doesn't teach almost anything useful for multiplayer).
The campaign doesn't increase difficulty gradualy as i should. Technically the "skill" cvar increases gradually but various other aspects of the campaign mean there's a sudden jump of actual perceived difficulty at the darkzone level. See
(spoiler i guess) the file which defines the campaign (0.8.2 version, switch to master branch to see git/autobuild but there weren't large changes). Darkzone is nothing compared to multiplayer with competent players ... though you will likely never meet so many of those at once
Basically bots are only good at DM, maybe a bit TDM, every other mode lets you win easily by just using your brain a little. CTF is especially bad. Most levels before darkzone are some "specials" which usually means humans with a bit of common sense are able to win easily. The only "normal" DM is oilrig which is a large map and bots don't have great pathfinding so they suck there too. On xoylent they seem to take way too long to switch targets, they're not very good at aiming to push someone off the map and they don't seem to aim for the ground to maximize the push so they're very weak; on top of that, the skill is set to only 6 even though previous levels got up to 8. Idk why, maybe an oversight? Darkzone then jumps to 9. I am not sure at what skill level bots start to time items but i think this is the first level where it's really noticeable.
And now the rant about DM: i think you can win most DM levels very easily by not really playing xonotic properly but just rushing everyone you see: if you kill him, good, +1; if you die, you lose a few seconds, respawn, and try again but don't lose points (unless you kill yourself). If you keep doping this fast enough, you can do it faster then the bots and "win" even though you're really bad at actually playing xonotic and you'd be useless at any mode which counts scores properly (in TDM your death means points for the enemy) or has proper objectives which can't be achieved by just shooting shotgun at everything and hoping for the best. On darkzone the bots pick up all the h/a (health/armor) they can get and go for some items *before* they spawn (might depend on selected difficulty) so the strategy to rush everything suddenly stops working. The bots might actually do dmg faster than you but even if not, they have enough h/a to kill you first. Being hard to see is imho just a cherry on the top, not something that actually causes the sudden difficulty jump. Try the level with fullbright players (megaerebus model) as the guide suggests and see if you can win then.
As for how to play DM properly - don't die so much, get vortex, RL and mortar (darkzone doesn't have vortex), learn to combo (again, guide) and keep your health above 150, the higher the better and don't rush the bots. Pick them off when they're weak or retreat. Notice their movement patterns and stay out of their way (i'd say being hard to see is worse when the bot is close but in general bots are good at close range and bad at distance).
<packer> when i see martin-t's name my blood pressure increases
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[BOT]Hоtdоg> anyone here lives near martin?
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[BOT]Hоtdоg> will pay monies for shooting him