Boss CR for group of 7, help!

[Stewbert]

So i've been DM'ing a campaign for about a year now its the longest consistent campaign we've ever done however news of the fun spread and our original group of 4 is now a group of 7 and im struggling to get encounters the right difficulty.

Heres my problem. The group of 7 average level 9. I've pit them against a CR 12 monstrous scorpion (12 vs 7x9 seems ok to me). The average party member has 20-28 AC the scorp has +34to hit. The grapple is +58 they CANNOT beat that, nor can they dodge any attacks.

This is what has happened, someone spots it coming towards them, about 100ft away buried in sand on the otherside of a sand dune. The rogue in the party thinking AC 34 is immense, charges it gets hit and grappled, then constricted immediatley and is nearly dead. This guy would probably feel cheated if I told him the scorpion hit his awesome AC on a roll of 2 and that his hard work boosting AC was for naught. The barbarian charges next he also gets hit and grappled but is ok with his huge HP pool, meanwhile the wizards of the group cast fly on themselves and the druid casts freedom on himself and runs away(helpful).

I see this ending in a horrible massacre, but can't help but feel like thats kind of ok because they charged into a 60ft scorpion with no tactics at all.

I'd just like to get some opinions here as i'm all for killing characters but only if its fair/deserved, I would feel bad wiping these guys out if they all think the boss is "too hard" but then i wonder is it really hard when fly + bow and arrow would have solved the problem. It seems like with a large group you need harder creatures, and with harder creatures no spells work and no AC matters, its just tactics.

I guess what i'm asking is, did I make the boss too strong?

Thanks.

[Hunter Noventa]

Grappling ALWAYS favors monsters versus characters, there's no viable way to build a character that can grapple anything but another medium creature.

That said, you might have made the boss a little too tough, but they were also overconfident and didn't use any tactics. They made some stupid decisions it sounds like, and well, stupid decisions should get you killed.

And when it comes to designing encounters, remember that Quantity is a Quality all its own, so you could pit them against several somewhat tough enemies instead of one massive enemy. one massive enemy encourages swarming and novaing, whereas several enemies will hopefully get them to think a bit more.

[Uncle Pine]

It seems like with a large group you need harder creatures,

With a large group you don't need harder creatures, you need more creature. A group with seven 9th level character has an effective party level of 10.6. However, pitting them against single CR 12, 13 or 14 creatures will either cause a couple of deaths per encounter or an instant victory by the party, sometimes both.
What you want to do to challenge a big group (heck, you'd probably want to do so even with only four players) is to bring the ratio of PCs:enemies from 7:1 to something like 7:5 or even 7:9. When they realize that they have to deal with multiple foes, players naturally feel in danger and start thinking fast. This usually lead to epic victories and a strong sense of accomplishment.
Having to build and maneuver several opponents in every encounter means that you'll have more work as a DM, but you'll have to bear with it.

EDIT:
Grappling ALWAYS favors monsters versus characters, there's no viable way to build a character that can grapple anything but another medium creature.
Kobolds.

[Aharon]

From what you describe, your group doesn't seem very experienced. Normally, this encounter wouldn't have been too hard - when the party uses abysmal tactics, that's not your fault. However, you could have given advice, i.e. "When you straight up charge it, you move through threatened squares and trigger attacks of opportunity". You could also point him to this table (http://www.giantitp.com/forums/showsinglepost.php?p=9556126&postcount=5), which gives average and maximum numbers of monsters - max BAB at CR 9 is 18, so there's about 20% chance to be hit by some monsters of his level despite his relatively high AC.

You might also have pointed out to your other players (wizards&druid) "Your tactic is pretty sound, but unfortunately, your ally panicked and attacked. It might be best to join the fight if you don't want to risk his death.

[turbo164]

Yeah as mentioned when building for a group of that size, it's often better to spread the monster's power out a bit. Two CR 10 scorpions, or a CR10 + four CR6 or something, would increase the chance of AC mattering and reduce the chance of instagibbing.

In a 4-man group the numbers are close enough that one big monster is easier to balance (and a rogue leading the charge would be a bit less suicidal at that point :P ).

[Inevitability]

Encourage knowledge checks, planning ahead and advanced tactics. Yes, when you see a scorpion the size of a large house and decide the best course of action is to charge it, of course you'll get slaughtered.

However, from the information your post gave me, I can see they could have easily killed the scorpion. Simply by casting Fly on a few people and having those lure the scorpion away from the grounded characters while they pelt it with ranged attacks could they have won this battle without anyone getting hurt. Note that while big and strong, our arachnoid buddy can't really hit anything flying a hundred feet above him.

I advice you to not rely on the CR system (which is broken enough) and instead pick monsters with care. If all they do is attack a monster in the most straightforward way possible, pick something that will prove an appropriate challenge when handled that way. Once they begin using advanced tactics, slowly start increasing the toughness of what you throw at them.

[Firest Kathon]

The rogue in the party thinking AC 34 is immense, charges it gets hit and grappled, then constricted immediatley and is nearly dead. [...] The barbarian charges next he also gets hit and grappled...
The scorpion will have to maintain the grapple, or drop the rogue to attack the barbarian. If it wants to hold the rogue while attacking the barbarian, it takes a -20 penalty (http://www.d20srd.org/srd/specialAbilities.htm#improvedGrab) on its grapple check. Till bad enough, but important to keep in mind.

I think that creature is a bit too powerful for your group. I wold rather go with two Gargantuan Monstrous Scorpions instead of one Colossal one. CR is one higher, but numbers look more beatable survivable.
Edit: Multi-Swordsage'd on the multiple weaker creatures part.

[Stewbert]

Alright cheers for the responses stuff to take on board, now to take responsibility for my actions and try save people from certain doom!

[Nibbens]

if you've never read this guide (https://docs.google.com/document/d/1nx-o8VAjhUwh3nnfzDQT-JA5eFLnN_BZJiBitGjBMDg/edit?pli=1), you need to.

[GoodbyeSoberDay]

I'll echo the "multiple enemies, not one big enemy" sentiment. Action economy is important, and generally for something to be a challenge to such a huge group it has to have numbers that fall off the RNG (as this scorpion does.) The problem there is that the beast can then very easily kill party members; if it couldn't, it wouldn't be a challenge.

I've only made the solo boss work a few times in 3.5, and it was always when the boss had ways of avoiding attacks and abusing the action economy without being able to one-round-KO anyone. A summoner, for instance, would be a way of making a "solo" enemy for CR purposes only.

[Crake]

The scorpion will have to maintain the grapple, or drop the rogue to attack the barbarian. If it wants to hold the rogue while attacking the barbarian, it takes a -20 penalty (http://www.d20srd.org/srd/specialAbilities.htm#improvedGrab) on its grapple check. Till bad enough, but important to keep in mind.

I somehow feel like, despite the DM probably being unaware of this rule (in my experience, many DMs initially are until i point it out), with a grapple of +58, i don't think -20 would have made much of a difference.