http://masterofmagic.wikia.com/wiki/Great_Drake
Those things are near unstopable. A single great drake enemy eats up 3 of my great wyrms or arch angels.

I was playing a nature/life mage who found a bunch of extra spells thanks to using great wyrms to loot a whole bunch of ruins

Great Drakes are obnoxious, but not as generally hard to deal with as Sky Drakes.

Web it and shoot it with ranged attackers can work. You'll have to spam Web several times, of course.

In fact, Web is optional. Great Drakes aren't all that fast, so if your ranged attackers are powerful, you can just kill it in the two actions you take before it reaches your units.

Well leveled high quality heroes with good gear can slaughter Great Drakes.

Top of the line normal units like Paladins, Berserkers, or Hammerhands can defeat Great Drakes, although there will be substantial losses. Who cares? Normal units are expendable.

If you have tons of mana to spend and/or some caster heroes with good mana pools of their own, you can just burn the Great Drakes down with direct damage spells. This isn't a great option for your magic schools, but it can work well for Chaos mages.

Great Drakes are quite powerful in melee (doubly so when attacking), so powerful ranged units are usually the best bet. If you have halfling towns then a group of elite Slingers is one of the best ways to take down Great Drakes (or Sky Drakes, for that matter), often times capable of taking down the drake in a single turn (2-3 high level ranged heroes with good gear can also get the job done). If you have to go melee then it's better to go with a large number of units that can swarm the drake rather than fewer but more powerful summons; this is to increase the instances where your units are attacking the drake and decrease the instances where the drake is attacking your units (because of its breath attack the Great Drake does twice as much damage when it initiates the attack).

And last but not least, flying units are a Great Wyrm's Achilles heel; they can't initiate attacks, and can only counterattack (and as you saw, get slaughtered). One additional advantage to Web is that it grounds flyers, allowing ground-based melee units to successfully attack them.

Previous posts are great. Only think I'll add is: Lightning Bolt is particularily effective. 2-3 full power bolts will kill one.

Depending on in combat casting to take down a Great or Sky Drake is a really bad idea. Unless you have Node Mastery most of your spells will fizzle and be useless. Secondly Sky D's are immune to your offensive spells even web.

The best way I have for killing either of those is with Hero spellcaster for ranged attacks. I give my trusty Malleus a +9 Atk +3 Hit and +3 Def Staff along with Heroism and he should be able to take out the Great Drake no problem. For the Sky Drakes he needs Jewlery with Movement bonus so he can run and fire in same turn but still keep distance.

Otherwise for either you need normal units with the ability to hit flyers, either by Flight/Chaos Channels or an attack such as Thrown that can initiate the hit to avoid their breath attack.

Won't their super high resistance value block webs and direct damage?
I know my great wyrms just shrug those off as the AI desperately throws them at me trying to slow them down.

And what archery units deal decent damage to them? they have so much HP and armor it takes a miracle to actually deal any damage. That might work against the 2 cities guarded by one great drake and a bunch of normal units. But one of the cities has 3 great drakes + peons. Stack size is maxed out at 9 unit.

BTW, great wyrms are plenty broken on their own. I rolled up several AI players with just 1-3 of those. Enemy attacking first doesn't seem to matter.

Anyways, it was late game, i researched everything, controlled all of arcanus and 1/2 of myrror. I had an army of 10 great wyrms. over 100 skill, 6 heroes with casting in the castle boosting my summoning rate. ~30k gold and 10k mana.

I ended up winning by passing the enemy's 5 great drakes and just taking his cities and eventually his capital

While I realize different playing styles appeal to different people, IMO this is why you struggled so much against the great drakes. Heroes with spell skill almost always have a ranged attack; a level 6+ hero, augmented with some decent magic items, can take down a Great Drake in two or three hits, well before the drake can close to melee range and fight back. Therefore, three heroes up = three drakes down; more if you use the simple trick of moving each hero one square back before shooting each turn.

Next time you play, try keeping one or two heroes back at your fortress to protect it and augment your skill; send the rest out as a stack, properly equipped, to do battle with nodes and troublesome enemies. See whether that helps.

Don't neglect low-level archer units that have been beefed up with experience, improved weapons, and possibly enchantments. Units that have lots of individual figures can improve dramatically since any of these bonuses applies to every single figure in the unit. For instance, earlier I mentioned halfling Slingers, which are great for this because each unit contains 8 figures. They're weak to start because each unit only has a ranged attack of 2, but those numbers dramatically increase once bonuses start being stacked on. Becoming elite grants +2 to ranged attack, along with +10% to hit (and slingers already have a +10% chance to hit from Lucky). And if you build them in a city with adamantium and an alchemists guild that's another +2 to ranged attack and +10% to hit. So at this point the single slinger unit has (2+2+2)*8 = 48 ranged attack strength, with each of those attacks having a 60% chance to hit (before range penalties). Then if you have any appropriate enchantments you can start stacking those on as well (such as Lionheart for +3 ranged attack, Flame Blade for +2 ranged attack, or Holy Weapon for another +10% chance to hit), at which point things just start becoming unfair (using this kind of setup I've had slingers 2-shot Great Drakes and Sky Drakes).

I double-checked the spellbook on this. Webs aren't resisted, only damaged (which takes just 12 hits; webs obviously don't have shields). But once webbed, flying creatures are grounded for the rest of the battle. Which not only means that they can be directly attacked by any melee unit, but they can be further affected by spells like Earth to Mud and Cracks Call. Great wyrms don't fly, they tunnel; so they're not affected at all by that aspect of being webbed.

I had a dozen heroes die on me in that game already and I killed dozens of the enemy heroes. I am aware you can make an uber hero but they were just too damn squishy AND too time consuming/expensive to make compared to summoning top tier units. A lot of summons do suck but the top tier ones (sky drake, greak drake, great wyrm, etc) can single handedly conquer enemies (not an exaggeration, the first AI I decimated in that game was killed by a lone great wyrm; he had killed another AI already, nearly killed me earlier, and controlled 3/4th of arcanus, that lone wyrm swept through his lands conquering town after town and eventually his tower... then his 2 replacement towers)

It is also worth noting that all together they added a significant amount of skill to my summoning rate and were instrumental in acquiring the army of great wyrms which utterly decimated the enemy players.

thanks, I will try webbing. 3 great wyrms being killed by a single great drake managed to reduce it to 25% HP from retaliations. Maybe if they are the ones attacking I could kill 1 with only 2 of mine lost.

Actually I believe you could do better than that.

Great Wyrm's have 25 Attack with a 60% chance to hit for an avg 15 damage. 45 HP, 15 Poison Touch, 6 Defense.

Great Drakes have 30 Attack with a 60% chance to hit for an avg 18 damage. Fire Breath 30, 10 Defense, 30 HP, 12 Resistance.

Now if you go into a battle with 3 of your Wyrms vs 1 Great Drake and you web it the first turn then proceed to attack it right away you should kill it the first turn without it getting to initiate an attack and thus rendering it's breath attack moot. None of your Wyrms should have more then 20 damage on them either. YOU LOSE NONE.

Actually it's odd but Great Drakes are tougher than Sky Drakes EXCEPT for the fact of the Magic Immunity.

Sky Drakes have 20 Attack with a 60% change to hit for an avg 12 Damage. Lightning Breath 20, 25 HP, 10 Def and MAGIC and Illusion Immunity.

Are you sure about this? My understanding is that the only two targeted spells not blocked by Magic Immunity are Web and Cracks Call, since they're not actually striking the unit directly.

Holy crap your right!!! WEB CAN BE CAST AT MAGIC IMMUNE CREATURES. I was 99% certain that it could not but to be sure looked it up on the wiki and lo and behold you were right.

I guess the reason I never found this before was because the only place I ran into Sky Drakes was in Sorcery Nodes and unless I had Node Mastery it was a waste of time trying to cast it. Just remembered I also ran into them guarding towers so perhaps I just never tried as common sense says it shouldn't work.

http://masterofmagic.wikia.com/wiki/Web#Usage

EDIT - Just want to add an apology for saying previously that web didn't work as I was WRONG. I mean it happens so rarely it's just hard to take in lol.

I once summoned a sky drake and had 5-6 books each in life & sorcery. So I cast invis and invulnerability on him. It turned out to be all I needed to defeat all enemies and win. :)