How do I build a robust army against an unknown target?

Against an enemy whose type you don't know (10+ tiles, beasts, wandering knights, wild dragons), forget the triangle's ±7%: it's random and not controllable. You build an army that "works in every case" by stacking only the controllable:

1. Your most researched type — your biggest permanent multiplier.
2. Green dragons for that type (+20% each).
3. A Center with a strong aura (a Champion like Rhysarion, -27.5% damage).
4. A single damage type plus its debuffs (e.g. lowering enemy Initiative boosts all your fire).
5. Troops filled to the max.

This army doesn't depend on "who am I about to face". It's strong by construction. The ±7% is just a small bonus you can only aim for when you know the type — which only happens on small tiles (below level 10).