Back to your initial query
Given a group of 8 characters (how do you manage them, I wonder ), the EL system is severely challenged - basically you could assume using twice the stated monsters simply to keep their CR. That is before taking into account the parties capabilities. So instead of one troll - make it two trolls, or two ettins etc.. The PCs' numbers are basically that of two standard groups, so serve them double portions of monsters.....
In general - look carefully at the party's strengths and weaknesses, especially tactical ones, when building challenging encounters. How mobile is each character, and capable to switch positions in combat quickly ? What are their weak saves, which are their strong ones ? How can their strengths be countered ? Spell-casting monsters are usually smart enough to recognise an arcane caster - which is hard to get wth willpower based spells, but soft as rotten wood resisting fortitude-based effects. Pick oposition accordingly and the CRs should fit - using twice the numbers, though. Unless of course a mulitude of monsters suddenly provide a synergetic effect which makes the _much_ more viscious. With opposition rated 3 or more levels lower, usually numbers make no big difference in the challenge rating, so you needn't bother to adjust the El for these, unless the sheer number (or some special tricks only available for a a huge group) poses a problem. Leader-type NPCs can be taken at normal, levelbased CR then. A horde of 30 orcs with two level five leaders would barely be an EL 5 encounter - and not much more difficult then say 10 orcs with two level five bosses.
Some ideas :
Higher HP-creatures are usually pretty good at countering "cleave" attacks - simply by not dropping after a single blow. Especially if they have damage reduction, fast healing or some caster backup with, say "shield other". "Fiendish" or similar templates can really up the ante on an old stand-by monster, say a troll with fire resistance becomes very much of a nuisance...
Give them mixed challenges - attack them with masses of orks, goblins or similar, using large melee weapons and ranged attacks (oil-grenades etc are wonderful foils for high AC ), and have a sorcerer assist them with web (I guess most of the guys have weak reflex saves still), some "ray of enfeeblement"s (no Save temporary Str damage - softens up fighters. Especially funny if the heavy armour suddenly makes them 'encumbered'...), "Blindess" (wicked on casters and rogues) or "darkness" spells, or a druid with "entangle", "blinding spittle" and summoned "nature's allies" to attack the caster types in the rear. Entangled/webbed/helpless PCs can be shot at at leisure by the ranged attackers.
Clerical "Silence", "Bless", "Hold Person" and "Doom" spells can hamper PC tactics and performance as well, without being high level. "Dispel magics" can do wonders against "buff maniacs" , too. NPC bards are great to bolster up a huge horde of lowbie "cannonfodder", too. Most of the humanoid races have leader-types noted down in the MM - simply exchange some of these with interesting NPCs fitting the race, but making things more challenging for your group.
Often enough, if the monsters hold (or take) the initiative (the tactical, not the mechanical one) in a battle and force the PCs to react and adapt to their moves, any fight becomes more challenging, even at the same theoretical CR.
As a general strategy, you can force the PCs to disperse (and then take advantage of the gaps to strike at the soft spots - casters and ranged attackers ) in order to avoid area effect spells hitting each and every one in the group ( a nice fireball into a close formation drives this point home easily enough - as does a "Snowball Swarm" or similar spell of level 2 or less). Similarly, a "silence" spell lobbed into a close, stationary formation (cast on a pebble say, thrown there with two dozen others as a grenade, to avoid someone actually making their save) can drive casters nuts and will hamper combat tactics some . Personally I wouldn't allow players inside a "silence" to share any tactical information with each other - if you feel mean, let them declare their specific reactions for the round written down only. After all, they cannot speak to each other anymore. all of a sudden a "standard" melee becomes far more challenging. And in dispersed formations, movement becomes all important - how fast can each character assist another ?
Alternatively, bunch them up in a confined area, where at best a few characters can fight (maybe around a corner or down a stair/ramp), and face tough opposition chewing on them. Making flanking and sneaking almost impossible and ranged attacks becoming harder because of cover and concealment....
Incorporeal monsters really don't care much for formations, especially if they can phase out of walls or rise up through a floor (like in a building ) to get at the rear echelons or simply do a hit and fade. Their touch attacks of course make heavy armour useless, too - so they would definitely be a fair challenge.
And remember - many monsters are quite smart, and capable of learning from mistakes and their observations. So if you see a nice PC tactic, memorize it/note it down, for use against the PCs in another session.
![:]](http://www.enworld.org/forum/images/smilies/devious.png "Devious :]")
) to turn gnolls into ogres. A gnoll with a battleaxe can deal 1d8+2 damage without a crit. An ogre is smashing out 2d8+7,with reach, and that is only a 2 EL bump. Go double from the ogre's 3, if that is a sufficient challenge for you party, and you encounter creatures like the 5 headed pyrohydra, capable of 5 attacks dealing 1d10+3 each. That is nasty. Now, not all high CR creatures are going to behave this way, but you must be careful with substituting bigger, nastier monsters, when you can add character levels to lower monsters and gauge EL that way.
