What should you attack to level up dragons fast?

If your goal is to level a dragon (raise its Reign Level), everything comes down to the target, not the army. Combat XP depends on the type and level of the target:

• Low XP: bridge attacks / tile defenders (a few thousand XP per fight).
• High XP: high-level tiles, Hedge Knights, wild dragons / beasts — and XP goes up with the level of the target.

So to level fast: go for high-level beasts / knights / wild dragons, not small bridge defenders.

On the army side: these targets have an unknown type → a sturdy comp (no ±7%), troops maxed out, aura in the center. And remember: Reign Level resets every season, combat XP is there to re-grind it fast (Meat is the other route).