Specifically being able to focus on a trait/design and sticking with it.
In general - initiative. Initiative is such a HUGE impact to every battle that gearing up for initiative improves odds/reduces losses dramatically. When I can add:
+3 from a lowly dagger
+1 from soldiers boots
+2 first turn from basic horse (other mounts have other effects)
+1 for belt of speed for 1 crystal
+2 via 'fast' trait
+1 per essence from a fortress (usually 3-4 for my main fortress) via aura of grace
+2 for strike upgrade
It isn't tough to get +12-14 initiative for mounted units or +10-12 for infantry.
Compare that to most designs that will bulk up on armor or initiative killers and it just widens the gap further. I'm not sure on the equation, but I know my 25-30 initiative troops generally have twice as many actions per battle as an enemy. Compound that with crushing blow or other effects with a delay and I can get 3-4 shots in per the enemy 1 (even if the enemy 1 is powerful).
Tie that in with dodge and I'll take my chances of dodging the 1 hit and getting in 3-4 per troop, where I can focus and take down the most dangerous foe.
Initiative alone makes it very difficult for mobs or the AI to eventually compete. Add in any other damage mitigation and it is very simple to avoid damage altogether - meaning your troops/heroes are doing more quests, getting more loot and seeing more of the map. While your cities aren't needing to churn out troops, they can be building infrastructure for the eventual wars. (Pretending the AI actually played) the AI is then stuck recovering or moving more slowly to accomplish objectives. Movement + initiative + dodge are easier mode.