Good idea but your math and assumptions are off.
For starters, the 12 headed hydra's attack bonus is +17 not +12.
Second, "barbarian with 18 con" is not a good assumption for the "meat shield." The game is supposed to be balanced for 25 point buy/default array characters not 36 point buy. Let's try an 11th level fighter with a 14 con--possibly modified to 16 with an amulet of health. Thats 11d10+22 or 33 for an average of 87 or 98 hp. We'll give him a 26 AC. (+4 fullplate, +2 ring of protection, +1 insight (ioun stone), +1 dex would be a two handed weapon fighter who spent a lot on defense; +2 fullplate, +2 large shield, +1 dex, +1 ring of protection would be a sword and shield fighter who focussed more of his items towards offense.)
So, in this scenario, the hydra hits on a roll of 9 and deals an average of 15 points of damage per hit. With 12 attacks, that's 12x15x.6=108 points of damage. Enough to kill the fighter stone cold dead in one round--even if the fighter is wearing the amulet of health.
Just for kicks, let's check out what happens to the "HPs are my defense" raging 18 con barbarian with a low AC (20 in this case--+3 breastplate, +3 dex, +1 ring of protection, -2 rage). He's got 11d12+44 hp +22 rage hp. 121 hp normally. 143 hp when raging. The hydra hits on a roll of 3--a roll of 2 if they hydra charges.12x15x.9=162 hp average damage (excluding criticals). The raging 18 con barbarian will typically be stone cold dead as well.
Now, let's take the Monster Manual at its word. Hydras are supposed to "reward characters with improved sunder." So, the characters have improved sunder and decide to close in and make a single attack against the hydra in order to feel like they're being rewarded for their feat choice. Oops. On the AoO, they hydra deals another 108-162 points of damage which means they're dead twice over. Some reward.
But maybe the role of the meatshield (and a very fun and rewarding one it is. . . .I'm certain of it) is to stand in front of the party, not to try and close in, not to try and attack the hydra--just to sit there and take damage so that he dies instead of the other characters (praying that they hydra is dead so that he doesn't take a second attack from all the heads--the given fighter will often survive one attack from the hydra if he goes to full defense (Against AC 29, the hydra deals a measly 81 points of damge per round--it'll then take two attacks to kill the fighter)). What are the odds that the 11th level party will kill the hydra in one round? (Assume standard single class adventuring party fighter, rogue, wizard, cleric).
Rogue: (assume he starts with bow in hand). Two shots from a +1 mty +2 comp shortbow. (Assume both hit). Damage 13.
Cleric: Flame Strike (assume the hydra fails its save). Damage: 39
Wizard: Cone of Cold (Assume a failed save). Damage: 39
Total damage: 91 points. It looks like the hydra gets a second attack. Bye bye fighter.
What if the party is a little more pro-active. But this time we won't assume all failed saves and all hits.
We'll give the fighter a +1 mty +4 composite longbow and three shots. (+15/+10/+5 for the barbarian--a best case scenario). Average damage per hit is 9.5. Against AC 22, that's 70%, 45%, and 20%. Average of 13 points of damage.
The rogue (assuming a 20 dex) attacks at +14/+9 for his 1d6+3. That's 65% and 40%. Average 7 points of damage.
The cleric's best bet is still Flame Strike. We'll assume a 22 wisdom for a DC 21 save. The hydra will fail that 55% of the time for an average of 29 points of damage.
The wizard really cuts lose--and attacks with a quickened Scorching ray (hit attack bonus is +7 (14 dex, +5 BAB) 90% chance to hit the hydra) for an average of 38 points of damage.
Then he tops it off with a maximized scorching ray (again 90% chance to hit) for 65 points of damage.
OK the hydra took 152 points of damage and is dead... as long as both the cleric and the wizard beat it in initiative. If either of them go after the hydra, the fighter is still a broken chew toy.
Of course, straight up damage may not be the best way to kill the hydra. Will saves like Charm Monster (even with the -5, it's still quite possible for the Hydra to fail), Hold Monster (good for one or two rounds of Held Hydra on average), Slow, etc might be better. And negative buffs are good approaches too. Ray of Enfeeblement +enervation (average 9 points of strength penalty and 2 or 3 negative levels) or even the lowly Doom could potentially reduce the hydra's effectiveness. Its mighty 12 attacks at +17 for 2d8+6 would be reduced to 12 attacks at +13 for 2d8+2 by ray of enfeeblement alone reducing the hydra's average damage per round against the fighter to 60 points of damage per round. (Hey now the fighter can survive one normal attack although attack + a single strong interp AoO still kills him). Doom would reduce that to +11 for 2d8--now the fighter only takes 38 per round (or per strong interp AoO). The fighter could then survive two attacks but not three.
All told, the creature isn't much of a reward for characters with improved Sunder. It'll just kill them. It's a creature that just plain kills any foes it comes into melee with--although if you stay out of melee, you can kill it without injury. Fine on a featureless plain, hopelessly broken if it shows up at close range in a dungeon, and just plain not fun all around.
Artoomis said:
Hmmm...
CR 11 = 12 heads. 2d8 + 6 for each head. Attack bonus +12. AC of 11th level "meat shield" ought to be above 20.
So let's assume that half the attacks hit. Average damage = 15*6 = 90.
That's pretty good! If that "meat shield" is a barabarian with 18 con (or more if raging), max fisrt hit die and average for the rest, he's got 12 + 10*6.5 + 44 hit points, or 121 hit points.
All he really has to do is survive one round of attacks. With no special defenses, it should be pretty easy for four PCs to do 139 (taking it to -10) points of damage in one round.
Total cost of the battle: A couple of attack spells and 90 points worth of healing magic.
Looks good to me.
Of course, it could be much, much easier for the PCs if they can say out of melee. This monster has no special defenses, so it is good in melee but pretty weak otherwise. PCs can get wiped out, but only if they make the mistake if thinking they can take this thing out in melee. In that case, it's tough.
