Get an Epic army yourself:

Make sure all 9 slots are filled with max size units.

Make sure those units are all upgraded to the best weapons/armor.

After assembling the right team:

Cast some buff spells on your units if possible pre-battle. Look through your spell list

Cast some spells against the enemy before entering battle, freeze is always a good one, lowers their initiative. Can cast a couple attack spells from strategic as well.

Other things you could do are:

Fighting many battles with the same army and gathering levels for the individual units, this *really* helps. You really need to do this with one hero per army as that is the only way to gather exp fast.

Softening up the enemy with a couple full size armies of units that you made just for this purpose.

When casting spells and doing multiple attacks against epic armies like this, try to do this all within one turn so they can't heal between turns.

N/A

Every person only falls for that once.

Everybody has their own strategy.  I like mages and a decently high level mage can easily take on a dragon in single combat, so I often do that and save my troops from the slaughter. 

If you need troops, I would not bring my best troops because a dragon can take out just about the toughest troops with one hit, so I often will use weaker troops that I don't mind loosing -- because against a fragon, you are likely to loose some troops. 

Spell resistance is key to surviving the fear, which is the key to killing dragons, so if I had time to make troops just for fighting dragons, I would give them traits and trinkets that improve spell resistance.  I'd also give them the biggest mauls I could find.  The bash ability is invaluable in dishing out the damage you will need to kill a boss monster.  But I probably wouldn't bother beefing up their health or defences and just plan on them being destroyed in one hit.  Any investment you make in armor/health is likely to be wasted if the dragon attacks since their attacks are strong enough to bust through all but the strongest defenses, at least against trained troups, where the dragons have the overpower ability that multiplies their attack by the number of troops in the unit.