Some of the CR / EL's are definitely out-of-whack, but I'd caution against raising the power level much more... CR 13 or 14 should be a pretty much top-end encounter for your group. Look at it this way... if the party met THEMSELVES in combat, they'd be an EL-14-ish encounter. (4 level 10's), and all things being equal they'd have a 50/50 chance of success (and very likely to suffer at least one death).
Some things to keep in mind:
1) The party shouldn't be able to rest up between every encounter. If the party is simply going to a dungeon or fortress, fighting an encounter, then retreating to rest --- the bad guys are going to make preparations for them next time, or may actually go after them and hunt them down. For a level 10-ish party, a single EL13 encounter holds a pretty good chance of them emerging victorious (but a high chance of death). But 8 CR-10 encounters over the course of 6 hours is going to be VERY VERY dangerous. They are likely to be seriously depleted in resources after the first encounter.
2) Play your enemies SMART. Look at their feats, and make sure they use them. A CR-12ish melee creature with power attack is likely to have an attack bonus to BURN, and can do insane amounts of damage with a single attack. Also, some of them have feats like Sunder. If they are a magical creature that requires magical weapons to hit, they can break magical weapons of that level of magical bonus or lower, and can often do it without breaking a sweat (check out the Earth Elemental). A magic-user is likely to have spell protections up --- very cheap protections like Shield are VERY effective even at high levels. A caster can also put together a small army in a heartbeat with Summoning spells. Remember - play your NPC's as if they were your own PC, seeking every advantage they can muster. Make sure they are appropriately paranoid. Intelligent monsters aren't going to just rush into battle silently and let themselves be hewn down. Guards will likely set off the alarm first, THEN attack. The evil wizard overlord isn't going to just sit in his study pondering ancient texts when his tower is being attacked. Use the same tricks the PC's use against them.
3) Have the party fight GROUPS of enemies, not just single monsters of high level. Have them work together if they are intelligent creatures. A single 12th level fighter isn't a huge challenge for a party of level 10's. But two 8th level fighters, an 8th level cleric, and an 8th level sorcerer - if they use teamwork --- could be very difficult, and though likely to lose a stand-up battle with your rested, fresh party, they'll definitely be a major risk factor. The two fighters, buffed with Bull's Strength from the cleric (among other things), ganging up on one party member at a time AFTER the sorcerer has opened with an 8d6 fireball and is following up with concentrating Magic Missile fire on the same target the fighters are attacking (while the cleric keeps everyone healed and protected).... assuming the fighters are using their feats to their advantage in the combat, the party is going to be having a fairly tough time. Remember --- in the wild, many animals DO use pack tactics. Wild wolves in the real world know how to flank an enemy. I imagine Wargs have it down to a vicious science.
4) Take advantage of the terrain / environment. A Pyrohydra in open terrain is a pretty easy challenge, but in an confined area (where its lack of mobility and limited attack range aren't such a problem) it can be devastating.
5) Make sure that the monsters are making adequate preparations BEFORE the combat begins. If the monsters are on the offense, they will have
One of the coolest editorials in Dragon magazine (from a very old issue, some time in the mid to late 80's I think) I ever read was entitled "Tucker's Kobolds". I'd recommend looking it up if you own the Dragon Magazine archive on CD-ROM (or if you have a friend who owns it, have him/her look it up and print you out a copy). It's about a DM who had a dungeon full horrible creatures that would turn their pack mules into fire-breathing demons or some such (I can't remember, it's been a while since I read the editorial). Anyway, a HORRIBLE dungeon, but the most horrible beasts of them all were the kobolds.
This was in 1st edition AD&D. The DM had taken SOME liberties with the kobolds I think --- some were casters, for example. But mostly they were plain ol' garden-variety 2-3 hitpoint Kobolds. Lots of them. But they OWNED that level of the dungeon, and they were smart and cunning and very very very nasty. They would shoot at the players through murder holes (think 90% concealment), toss molotov cocktails of flaming oil flasks into the room with the party (area-effect doesn't require a great "to hit" roll), push barracades of flaming debris towards the players with poles... The players learned to fear the kobolds more than the fire-breathing demons. Those pathetic, dime-a-dozen kobolds just SHREDDED the party out of sheer cunning and numbers.
And as this editorial was written by one of the players, it was evident that he LOVED being terrified by those pathetic kobolds. It was a memorable, completely enjoyable experience. A good "bad guy" does that to players. Something to think about....
(Edit: Did a little research - The original editorial was apparently in Dragon issue # 127, and there was an article inspired by that editorial that appeared a few years later in Dragon Magazine # 174, entitled "Defeating More with Less".)
