For a face to face brawl with a chambercoil dragon ( the weakest) level 10+ on 2 heroes, some archers, one unit to tank and a few hard hitters (infantry, cavalry with blunt weapons) should do the trick.
Obivously if you have Juggernauts it's easier 
Does Shrink works on Dragons? damn that's crazy !
Don't forget to use spell on them if you can before engaging one (freeze, tremor) to lower its initiative for exemple.
Anyway, try to avoid settling a city close to a dragon lair, even if the location is appealing, you might unleash something you don't want to deal with early on.