I have
1) Swadian cavalry, Tier 5 & 6 (30 percent)
2) Vaegir archers, T5+6 (25 percent)
3) Rhodok crossbowmen, T5+6 (25 percent)
4) Nord infantry, T5+6 (20 percent)
and usually lose 0-5 soldiers in battles against size comparable armies.
Only when I fight against Khergit, my soldiers drop...