I just finished one of the hardest czech mods for half life (Half life: The Real)
What made this mod so hard? It's because it's a partial conversion with custom weapons and monsters. In addition to that, the monster and weapon stats have been beefed up so that everyone dies much faster. One of the most common death is getting hit by a headcrab. Get hit once and your dead, its extremely realistic because if your just a random guy, a headcrab will kill you in 1 hit because you got your head headhumped.
What's cool about this game, is that it's actually 2 games in 1. The training course is itself a 30 map mod, similar to residual point. Not only that, the 'training' mod and the actual mod involves different weapons and monsters too. The training mod is closer to the vanilla experience where you can get killed by regular headcrabs, soldiers, and other xen aliens. Meanwhile in the actual mod you're in a space station invaded by all kinds of headcrabs, and blue vorts. Yes, there are several different kind of headcrabs, akin to headcrab frenzy. I'll even tell you what kind of headcrabs there are, because the reality is they can all kill you in 1-2 hits.
I played on hard so these are the damage values:
Regular headcrab - 200 damage (can survive a hit with full HP and armor)
Red headcrab - 400 damage (1 hit kill guaranteed)
Blue headcrab - 80 + flashbang your face for several seconds
toxic headcrab - 40 + poison damage over a long time
purple headcrab - you get healed a large amount
Most of your deaths will be from headcrabs, especially the red ones. However, the developer was kind enough to introduce other ways for you to get killed. Not including drastic buffs to most of the vanilla monsters, there are some new guys here to wreck you a new one.
Black zombie Barney - 150-175 per slash + flashbang your face
Blue vortigaunt - charges up within half a second, 300 damage
nuclear tripmines - they dont emit a lazer so you need to look around very carefully.
Otis - Hes always spawning around a bunch of headcrabs so he never lives long enough.
Green Gonome - 30,000 HP. I didnt want to get hit by this thing to find out what it did.
One of the most annoying places in the mod is where you ride this waterfall out of a cave, where you are immediately attacked by 2 helicopters. You must beeline to cover because the area you come out of is also covered by a 6-barrel turret AND an automatic rocket launcher. I think this was the hardest part in the entire mod, moreso than any headcrab ambush (and there are loads of those)
In fact you can even get killed right after the opening cutscene. Soon as you spawn into the game a bomber jet casually flies overhead, then the area you are in is bombed. That was my first death in the mod, soon to be followed by many, many others. I lost count.
FEAR NOT, though! The developer has introduced lots of new weapons to counter the increased power levels of the enemies. Among the vanilla weapons, you can also get:
A five seven pistol from CS
Colt m4a1 from CS
SG550 from CS
P90 from CS
Knife from OP4
shotgun from gunman chronicles, set to highest bullet amount (its alot)
sniper from OP4 that sounds like the CS AWP
deagle from OP4
M41A from azure sheep
shockroach from OP4 (i didnt get a chance to use this)
Try this mod now, if you love to get killed lots of times. I certainly did.