My attempts at recalibrating the MM2 beasties

[Runestar]

It seems that the disparity in cr becomes all the more extreme at higher lvs, so I will try to focus more on the supposedly more powerful monsters. The linnorms are up next.

General observations: The designers seem fond of giving dragons powerful spellcasting abilities. While this does undeniably make them stronger, it also has the unintended side-effect of making them less draconic in nature, and more like casters with more hp, as their already weak physical attacks cannot keep up with their new cr boost.

The linnorms appear to suffer from having too few HD, so they are still fairly fragile spellcasters. As such, they should never ever engage in melee, instead relying on their spells/breath weapon to whittle down the PCs. Also, the assumption here is that you use their spellcasting like an intelligent spellcaster would (eg: long duration buffs already in place, buff extensively prior to battle). They are thousands of years old, after all.

The dread linnorn has made an appearance in dungeon 149, where its cr was revised downwards to 21 (or 22 or with the elite array). This was in addition to it receiving inherent stat bonuses via wish and +4 enhancements to its various stats via extract gift, making it significantly stronger than its 'naked' MM2 entry. In addition, the swamp linnorm has 22HD, druid17 spellcasting and is cr24. I shall be using these as precedents.

Gray Linnorm: Cr16. Taking into account its low HD, I will comparing it to the planetar (MM, cr16, cleric17 spellcasting). It lacks the planetar's glut of SLAs, but let's be honest, you won't have the opportunity to use half of them in battle anyways. Make sure you give it quicken spell so it can cast 2 spells/round.

Dread linnorm: Cr21 (as stated above). Arcane spellcasting is so much more readily amusable, so I will leave its tactics to your imagination.

Corpse tearer: Cr23-24. My biggest reservation was its cleric17 spellcasting, as this doesn't really give it a lot of high lv spells to throw around. Compare with the solar, at 22HD, cleric20 spellcasting, cr23. It does have 28 HD though, making it significantly more durable than the gray linnorm and more importantly, allowing it to qualify for the epic spellcasting feat. One slot can be used for epic mage armour (+20AC), giving it more staying power. Other 2 slots, perhaps contingent resurrection, a custom spell granting +16 enhancement bonus to a stat of your choice, and an attack spell?

Though at this point, PCs should be more or less immune to attacks like paralysis, grapple, energy drain and to some extent, spellcasting (between high saves, blanket immunities and ample ways of countering/negating their effects). So don't get too hung up over those.