Which troop should you pick when you don't know the enemy in Dragonfire?

When the enemy type is unknown — level 10+ tiles, beasts, wild dragons, Hedge Knights, which is most of the time — don't try to guess the matchup. The opposing type is random: you might land on +7%, −7% or neutral with no way to predict it.

The right approach is a matchup-proof build:

1. Pick the troop type you've researched most, the one that fits the most of your dragons on green affinity.
2. Get the Center right (your best aura to spread).
3. Optimise the dragons, the +20% affinity, Initiative and the stat debuffs.
4. Forget the ±7%: it's ±7% you don't control, when a good aura is worth −27.5%.

In short: against the unknown you maximise what's controllable and ignore what isn't.