So my players easily passed throught our first game session, slaying easily the cult followers and the sorceror of tzeentch effortlessly. So four the second adventure, they will enter a mausoleum where dwells not nly cultists, but deamons and mutants. But how to make certain they aren't going to hit the wall? In the first game they were able to overcome encounter with a difficulty rating of 6 skulls, so I thought maybe putting challenge with a higher rating like 9-10. Is it a good option, or should I keep the same rating and smple put less creature but with a higher difficulty rating?

From my experience you really can't be sure, in a way it's all about a gut feeling (at least for me) and I seldom use the difficulty rating when I GM, over the sessions I've just developed some kind of intuitive feel for it.

If you want to experiment with difficulties a tip can be to have a backup plan going, if the fight isn't going very well you could let the players roll observation (or hint that they should use assess the situation) to notice that they can collaps a section of the wall over the cultists/mutants/deamons, since the mauseleum is in bad shape. Or just collaps the wall yourself and let the players roll to dodge it (while burying some of the enemies).

I generally do not want to "save" my players, I want them to feel that they can die, but when we started playing and I was unsure about an encounters difficulty I generally like to build a back up plan into it. A way for the players to tip the scales in their favour, or just get lucky. A total party whipe out is never fun, and might end the game then and there. So I try not to do that.

Tweaking the difficulty to make it harder during the encounter is easier in my oppinion. One of the easier tricks is using henchmen, be ready to throw in one or two groups of them and let them come from behind (or from a side passage) in the middle of the encounter if the players seem to have a too easy time of it and the encounter might grow much more difficult. Or you might let one of the cult leaders flee when he/she sees that his/her side is loosing, and then come back with reinforcements (that way the players can actually try to reach the leader before he gets away, but this would require them to perform stunts, dodge or otherwise get by all the other cultists in the encounter and that might be dangerous in itself). In my experience this can make a combat a very rewarding experience, as unexpected things happen in the middle of it which changes "everything".

Hope it helps and good luck with the encounters!

*scratches beard* Something seems a bit off. The threat rating (skulls) shown on a monster are usually (at least in 2nd edition) intended to match up monsters as they compare to other monsters, not to some PC threshold (challenge level adjustment has always been a bit arcane and GM fiat-y in all editions). However, if you were using creatures in the 6 threat rating range on a group of new players it would be good to know a couple other things:

How many players do you have? Did they start out with by-the-book generation, or are they advanced to higher rank? Was there a significant location or situational advantage for the players against the cultists and sorcerer? Did they roll uncommonly well, ie everything coming up comets? Are they in possession of some powerful artefact or similar thing? Did the combat simply take place in a square room with 10ft cielings with all the participants starting at close range?

IF the individual threat rating of your encounter was something like 2 cult followers (1 skull each) and a chaos sorcerer (4 skulls) then that could make a little more sense, but I would still be curious as to the above questions to figure out how they completely depantsed a sorcerer and his goons without so much as a popped pimple. Not thinking you've done anything wrong per se, but I'd personally like to know how I can avoid a similar situation if it is really just the game mechanics playing out here. I'm used to magic using enemies scaring the pink panties off my players...

If I have to throw a lord of change at a rank 1 party to kick their teeth in then I've got some major house-ruling to do (ie stealing all of Gallows' ideas and claiming that they are mine).

I have to play echo to Callidon here.
6 skulls rating would have my party cowering in a dark corner. (4 rank1 characters, 2 casters, 1 ranged & 1 melee)

In each case: not too difficult, could be done by the GM himself... Intuition checks as mentioned before could be a good idea. You could opt to NOT use a certain power. In the end, it's really cooperative story-telling, so if you're telling a story, and something turns out way too deadly, or your whole party keeps rolling crap the whole evening, there's nothing saying you can't even the odds a little (or beef them up in the reverse situation).

Oh sorry, I shoudl have been more specific. So the party include a dwarf Iron Breaker, a human bright order apprentice wizard and a human initiate of sigmar. Their first encounter was against 3 ruffians. Their seconda was against 3 cultist, and their final was against the sorceror who was alone (so 3, 3 and 4). The thing is that the Iron Breaker has a really strong defense and soak value and the iniate often use Sigmar Hammer with a great hammer, so usually he scores more than 7 damages (which is sufficiant to take down a cultist). But now with all our advices I think I get it.

Defeating a Sorc of Tze is easy, if the players are allowed to focus on him, if you had swapped that sorc with a melee Dificulty 6 npc/monster, I am certain they'd be very very hard pressed.

Dificulty 9 or 10 are demi-gods, and I can't see any way rank 5 or below characters could take on those. They're monsters that take down characters in sinlge hits. Some of them will deliver 20+ hits without much trouble, add then 3+ critical hits.

One thing to remember though, is that when you play npc's like Sorcs of Tze, who has high Int, you should player them smart. Have them be disguised as a normal thug, have them maneuver around, have them use their most nasty and best spells/actions to kill the PC's.

To play monsters/npc's at their right Dificulty rating, you have to WANT to kill the PC's. So you have to work at your very best to kill them. That means ignoring the Iron Breaker, and take down the wizard, then the Priest, and finally swarm the Iron Breaker. Of course this implies their opponent being smart, and not a troll for example.

Oh I've discovered in part why the encounters were not as difficult as much as they should have been (idiot that I am)! When the creatures were dealing damage or receiving hits, I forgot to add their damage value to their Strenght or Agility and to add their soak value to their toughness! Beginners bad luck....