I would go for well-armored or high-initiative archers as the mainstay of my army.
If I go for well-armored archers, then I want to firstly cast hast on my archers and secondly spread them out so that the dragon can pick one to beat up for a while, then go on to the next. I would also try to slow the dragon after hastening my archers, but only if 1) I think there is a reasonable chance that I can slow the dragon
If I go for high initiative archers, I would also include a fast, high-initiative, low-health melee (or maybe ranged, but I want that initiative) unit for the dragon to chase around for a while while the archers turn the dragon into a pincushion. I would still scatter the archers, but I might try to slow the dragon before hastening the troops if I think there is a good chance that the spell will stick. As for why the melee unit is low-health - I don't intend for it to fight the dragon, I want it to distract the dragon, and the AI seems to prioritize low-health targets over high health targets (which incidentally makes your supporting champion a reasonable candidate for this role, since it probably has less health than the archers and, since it's a champion, can probably be made to achieve a higher initiative than most of the stuff you can train for less than the cost of training anything - just get a dagger from somewhere, and anything else that boosts initiative). Thus the melee unit, if it is a trained unit, will probably be a minimum-size group on horses (for the movement) or wargs (for the initiative) armed with daggers (for the initiative) and equipped with a minimal amount of armor (if any) to avoid any encumbrance penalties.
Technologies that I would want: some form of ranged weapon, whether a staff or a bow, and the higher tech the better; a mount, probably a horse for the mobility, but wargs are good to counteract the initiative penalty on a bow; and some form of armor. Larger group sizes are also better, as your troops will deal more damage that way, but it isn't necessary and may make it more likely that the dragon will go after your hero first - which I would generally want to avoid with a supporting-type hero. Any technology that gives equipment
Note that it is possible to beat dragons using troops equipped with only leather armor (or Robes of the Aegis), spears, and horses - I've done it, but I put it off until I had the 3rd of the four group sizes and wasn't all that concerned about losses, as I had about 20 cities and had focused research in the Civilization tree and thus had a large income with which to replace losses. Lower-tech armor (or even no armor, though that severely limits the utility of your troops against most other things) may even be better than higher tech armor, because you don't get the same initiative penalties in leather that you will in plate, and plate doesn't do any more against a fire attack than leather does (assuming the dragon uses the fire attack) - acting more often is better in this case than taking less damage from melee/ranged attacks, because 'less damage' only leaves you as 'mostly dead' instead of dead, and you may do better by training lots of weak sacrificial troops and sending armies off as if they were attack waves to wear the dragon down than sending that really expensive army you spent all game and all your resources building off to get killed by the dragon (you could also combine the tactics).
Essentially, the minimum tech I would need is 'None' - I just need to have enough rapidly-trained, unsupported, weak troops to wear down the dragon or otherwise occupy it until something of mine can actually pose a legitimate threat to it. Magnar works really well for this as you can train up a bunch of cheap slaves quickly (preferably armed with spears or better, but anything that can consistently do at least one damage which you have in enough quantity to deal more than the dragon's level worth of damage each strategic turn will work - I like spears because they ignore some of the dragon's defense and as such are much better than any other early weapon against high defense - they probably still only deal fractional amounts of damage per spear, but it's a larger fraction). More importantly, slaves cost you nothing except time (if you use the build queue) or a single payment (if you rush them), and this can be used, with a sufficiently high-production city or a sufficiently low-cost slave, to make two slaves per turn from a single city - rush one for the miniscule cost (relative to a real unit) of a slave, train one properly in the city queue, repeat on next turn, and if it doesn't complete training on the next turn, rush a slave that hasn't begun training yet, since you won't lose any production time spent on the incomplete unit. This can be a bit expensive, but it will certainly speed your troop production and works with any unit - it just takes longer with most normal troops and is more expensive, and if we're sacrificing units to kill a dragon I want those units to be cheap
Considering low tech (and, possibly, low-level champions):
Useful Spells - Curse and Graveseal (in Death Magic) are also great against a dragon (assuming you can get them to stick) if you've got low-end troops, because they increase the damage suffered by the dragon (especially Curse). I would not use anything out of Life Magic against a dragon if I have low-end troops, because Shrink just makes the dragon harder to kill without significantly improving the survivability of my units (Yay! I'm half-dead instead of all dead! Heal me so that I can die on the next hit anyways!), Heal is useless if a) the dragon can kill your units in one hit or 
can kill the units in two or three hits even if you heal them, and Growth is only marginally useful on low level weapons, since low level weapons have attack values ranging from 4 to 9, add 50% and we get 6 to 14, which is still much less than the 20+ defense of a dragon - although it is more useful than anything in Death Magic aside from Berserk (they are going to die in one or two hits anyway, who cares about the 1-point of damage per turn?) unless you have a high enough spell mastery to get the curses to stick to the dragon.
What magic to have on the supporting Champion? Air. No other school can give you as much with a low-level caster as Air can if you face a dragon. Haste your entire army as soon as you can, spread your forces out so that you can get the dragon to run around in the middle after something fast in the center if you have ranged troops or sacrifice your melee troops individually to the dragon. Life + Air might work if your troops can survive a hit and you can rotate weakened units out of the line fast enough to keep them from dying and long enough to heal them to full health, but it will cost a lot of mana and you'll need Wellspring on your mage to do it properly - heal is alright if you only need to rotate between two or three units, but you probably don't have troops strong enough to go toe-to-toe with a dragon for more than one or two attacks by the dragon, even late in the game.
Unless you have a high amount of spell mastery or are very lucky on your spell resistance checks, curse-type spells (Slow, Wither, Curse, Blind, Shatter) and direct damage spells (unless you have a lot of the shards which increase the spell damage) are wastes of mana, because the dragon will either resist the spell (which wastes the mana for curse spells and halves the effect of damage spells) or it won't matter much anyway - Curse lasts three turns, but the dragon probably has enough health to survive those three turns and you probably blew all your mana getting that first Curse to apply, anyway, and once Curse ends you're back to doing about 1 damage each time a unit attacks since you don't have good weapons; Blind and Slow prolong the battle but won't save you in the end, though they might allow you to weaken the dragon more than you would have otherwise; Wither, unless you have a lot of Death Shards isn't going to significantly impact the dragon's attack value; and Shatter is a relatively high-level Earth spell that costs a fair amount of mana, does a little damage, and reduces target armor slightly - but since your weapons are crap it doesn't matter, because if the dragon had 20 defense before being Shattered, it has 15 afterwards, and your weapons are not that much better against 15 defense than 20 - I think you're better off hoping that Curse gets gets through the spell resistance than that Shatter gets through, unless you've already lost most of your army. Damage spells are equally useless because you will probably go through most of your mana to deal one or two damage per cast, which you could have done by whacking the dragon with that dagger you found in the spider nest and the result would have been the same but at a much lower cost (assuming we have a low-level champion and few shards, or a high-level champion without Flame Dart without many shards backing him up). Damage spells and Wither become much more viable if you have the shards to back them up, because one-half of whatever large amount of damage you would have dealt had it gotten through resistance is still much better than the one or two you'd deal with normal attacks, and a Wither lucky enough to stick to a dragon when you have a lot of Death Shards might actually be worth sacrificing the champion to cast it (and you probably will lose the caster - that casting time immobilizes the caster for two turns, and the caster is probably the thing on the field with the least health, so is likely the target of the dragon). Conversely, curse-type spells (other than wither) and Flame Dart become much more viable with high level champions as the curses are more likely to stick and Flame Dart does more damage, which is more likely to deal its full amount since your spell mastery is likely higher. Anything with a casting time is probably not worth casting (this includes Wither) unless you have a high spell mastery (to make the spell affect the dragon at its full value) and a lot of shards of the type that make that particular spell better, because any spell with a casting time probably means that you have one and only one chance at applying it to the dragon, and then your champion is dead for the duration of the battle (if you took Path of the Defender, then I don't think your champion really counts as a support-type champion, unless you count the army improvement skills - Path of the Defender is more the 'I want to go toe-to-toe with anything the world can throw at me' path than the support role).
Elemental Forums
can kill the units in two or three hits even if you heal them, and Growth is only marginally useful on low level weapons, since low level weapons have attack values ranging from 4 to 9, add 50% and we get 6 to 14, which is still much less than the 20+ defense of a dragon - although it is more useful than anything in Death Magic aside from Berserk (they are going to die in one or two hits anyway, who cares about the 1-point of damage per turn?) unless you have a high enough spell mastery to get the curses to stick to the dragon.