Alright, so i'm running a Dark Heresy (why else would I be here). and i'm just having a bit of trouble making things difficult for my players. Here's my players. They are all Rank 3 with 1200 spent exp.

Cleric - Knows the most about 40K out of the players, has the most wounds, toughness and armour. I can't remember his wounds but he has 4 TB and Guard Flak armour for 4 armour, so 8 damage reduction all over. He uses an Autogun and mono sword, also has some grenades and fire bombs.

Assassin - Has a general understanding of the 40K setting. Stays 51m behind the other 2 with his long las when he can (So he is out of effective weapon range for most things), when indoors or close quarter situations, he duel wields auto pistols. He is also is incredibly smart in real life, so he can usually find pretty clever ways to deal with any situation.

Psyker - Knows next to nothing about 40K. Las Pistol and 5 minor psychic powers, generally spasms things and then takes pot shots at things. Usually doesn't take things too seriously.

So i've run a few of my own missions and I can never seem to challenge my players in combat. When I think i've got things challenging enough they just breeze through it.

Since I didn't have time to write my own mission, I decided to run the "Edge of Darkness" campaign and modify it to up the difficulty.

So far they talked to Lili, they didn't really ask much, but did learn about Evard Zed. After talking to him they went to the workers union (The assassin got in a fight with someone) and the psyker managed to dig up the information on the Narcogangs.

I decided to kick the night cycle into effect, so they headed into the hostel for the night. Taking turns keeping watch, the assassin managed to notice some body snatchers eyes in the darkness, so they decided to follow them, (not knowing what they were yet) using the Psykers sense presence to guide them through the dark. They followed them to the statue on the map, there were 5 of them. When they were 15 meters away they turned on there glow lamps and threw them over to get a better look at them (I thought it was quite smart), they all passed the fear test.

The Cleric threw a fire bomb (plus a frag next round) that lit all 5 of them on fire, some shots fired by the assassin and a successful spasm. It was at this point I realised how slow the body snatchers are, plus the breath leech from Psychic Phenomena meant they could only move at 2m for the first turn. Long story short, only one body snatcher got an attack in, but the cleric dodged it.

Could you guys give me some ideas on how to change up "Edge of Darkness" and make my players feel the pain of Dark Heresy while still being a fair GM. I was thinking of giving the body snatchers hallucinogen grenades or make their attacks cause the hallucinogen effect.

This always ends up being an issue.

First point, I don't let anyone but guardsmen get Guardsmen flak, other than that the Cleric isn't doing anything off.

Assassin: Sniper fight, make him move if he wants to play the metre by metre game.

Psykers will take care of theselves with Perils.

However they seem to be playing perfectly well within the rules. I mean my acolytes decided to launch an attack on the capitol building of a planet because they didn't want to wait for an appointment....

However Edge of Darkness is an introduction adventure for a reason, it's not meant to be high power. However if you have Disciples of the Dark Gods there are some interesting things the Logicians have up their sleeves. Creatures Anathema has some tech heresy/hive mosters that you can throw at them as well. Nothing beats a smart enemy though, if the acolytes want to play smart then you have to play smart as well.

Yeah, Edge of Darkness really IS an introductory Adventure for starting-characters. Anyway, something I usual notice on myself: All the nasty stuff the players use? You can use them yourself. Overwatch, Suppressing Fire, Aiming and the various talents? NPCs can totally use that. Oh, and being 51 Meter behind the other Guys just means enemies have it a bit harder to hit him - he's still in range.

If they're doing this on some gang's turf, it'd be hilariously easy to add a third party to the firefight, because frankly...grenades are really, really loud.

You should give this thread a good read.

Yeah, Edge of Darkness really IS an introductory Adventure for starting-characters. Anyway, something I usual notice on myself: All the nasty stuff the players use? You can use them yourself. Overwatch, Suppressing Fire, Aiming and the various talents? NPCs can totally use that. Oh, and being 51 Meter behind the other Guys just means enemies have it a bit harder to hit him - he's still in range.

More importantly if an enemy outflanks or just emerges from behind his buddies are that much further away to help him.

BTW Something I learnt very early on is that you don't need to make the win conditions of a scenario defeating the bad guy in combat. The enemies they face could be horrible and well out of the league of the Acolytes and the scenario is to escape or gather information without being seen. On this basis don't be afraid to really mix it up by throwing in something that the PCs can't possibly be expected to handle in a straight fight and watch them get creative (or just run!).

Two examples I have used;

1) An Eversor assassin on a rank 2 group. Acolytes are protecting a dignatary that the eversor wants to find and kill. Hilarity (and by hilarity I mean terror) ensues.

2) A Chaos Terminator on a rank 4 group. Acolytes known the mutants are worshipping soething i nthe swamps, investigate and discover a possessed suit of Terminator Armour. They need to find the ritual to banish the spirit while the machine ramapges through the country side.

Both of these were really fun scenarios because the PCs knew they couldn't just blast the enemies in a fair fight.

One of the best sessions I'd ever played, was based on Edge of Darkness, instead of body snatchers, the heretics were trying to build space marine like creatures. we spent the bulk of our session trying to sneak into the Alms house, only to find one of the monsters, and then have to run away, and try to lose it in the streets. It was so much fun, and probably is the reason that I GM now.