Which army composition for which target in Dragonfire?

It all depends on whether you know the enemy's type or not:

Scouted target / known type (tiles below level 10, PvP you've scouted):

• Pick the troop type that counters the enemy on the triangle (+7%).
• On top of everything else (research, green affinity, aura).

Unknown target (level 10+ tiles, wild beasts and dragons, wandering knights, held points — the most common case):

• Ignore the ±7%: the defenders' type is random, so you're as likely to be +7%, -7% or neutral.
• Optimize everything controllable: best dragons, Center aura, +20% affinity, type research. A robust comp that doesn't depend on the matchup.

The reflex: ±7% only when you're scouting a small target. Otherwise, don't guess — maximize what you control.