If we are talking wargame then below is my analysis of Lores for methods of dragonkilling(only damaging spells, buffs/debuffs are not included):
Fire: no herokillers. Lots of AOEs, but all of them are S4 (for comparison sake it is the strength of a bullet). Can theoretically kill a character who stands alone instead of joining a unit by a swarm of S4 missiles. Completely useless against T9 W9 Emperor Dragon.
Beasts: Amber spear is like a ballista bolt, overcasted version is cannonball. With Multiple Wounds (D3) both versions are decent herokillers, latter is good monsterkiller. Can kill Emperor Dragon in ~5 casts by crunch, likely sooner in-quest with BoneyM's battle magic scaling (or not, since fluff-wise I can see super-dragon tanking several of those).
Metal: Very good at killing armored targets, since metal spells ignore toughness and scale with target's armor. Both Searing Doom and Golden Hounds would be effective against Emperor Dragon (about 3 casts of overcasted Searing Doom should be enough). The only concern is that BoneyM may rule that "natural" armor doesn't count for spells of this Lore (in such a case it becomes useless for dragonslaying).
Light: Banishment scales with the number of Light Battle Wizards in the vicinity of the caster (fluff doesn't say why, my guess is some kind of collective channeling). Can theoretically become dragon-killer with 6-7 Light Battle Wizards, but useless against him on an item. Most vulnerable to dragon's dispel since he is a Light wizard himself and knows intimately how it works.
Life: Dwellers Below is an insta-kill regardless of Toughness and Armor, but it allows Strength save and thus useless against S9 Emperor Dragon. Fluff-wise probably shouldn't affect flyers anyway.
Heavens: Lots of cool spells and Lore Attribute would be useful against lesser flyers, but Dragon's T9 no-sells all of it. (Normal dragon would be badly injured by Urannon's Thunderbolt, but this one will be barely scratched).
Shadows: Penumbral Pendulum is S10, D3 wounds and thus comparable to Amber Spear. Pit of Shades is insta-kill with an Initiative save and Emperor Dragon has Initiative of 1. If it gets hit before it can buff itself with Speed of Light (granting him I10) it is dead.
Death: Spirit Leech is a monster-killer but it works through Leadership contest, and Emperor Dragons have shounen willpower, making it useless. Caress of Laniph and Fate of Bjuna will likely cause zero hits (it is 2d6-S and 2d6-T respectively). Purple Sun is an Initiative-save insta-kill like Pit of Shades, but persistent moving vortex instead of direct damage.
Conclusion: Surprise Pit of Shades or Purple Sun are our best bets. Metal is good if he is vulnerable to it, Light requires a squad of Battle Wizards. Pendulum and Amber Spear are no better than mundane cannons, everything else is trash. If this dragon is a caster (he likely is) then he probably knows Boon of Hysh which is a full heal, thus a battle of attrition is very dangerous.