"Eternal sleep"
Imho, easiest way is get dryads (with other 1-3 lvl troops without immunity to sleep) and have "gift" spell and Agni ring. Best is all elven she-fighters (because of ring your troops will move before enemy and they have no retaliation) and some shooters.
Tactic is simple:
1. Dryads which uses sleep ability.
2. All units "wait". (turn 1)
3. At end of turn attack enemy with spirits/fairies, shooters, magic and rage spirits (just dont use abilities with DoT effect). (turn 1)
4. At beginning of turn 2 use "gift" spell on dryads if needed. (turn 2)
5. Attack enemies with units which have larger iniative than dryads. (turn 2)
6. Dryads uses "sleep" ability. (turn 2)
7. Pass till end of turn. (turn 2)
8. goto 2
Result: No casualties (your "double" cannot use magic because his units have no move).