lool.
best way is : archers to reduce Acheron's health before combat (any boost to rise the damage limit is welcomed -many archers if 1 archer can't do it in 1 turn)
the strike with 1-2 beastmaster boosted to max : poison blade, combat + subdue animal + subdue beast + spider blood (+1poison) eventually with elephant blood (+1combat) (orthus Axe or bow of the long sighted or any other +1+2strength are also welcome, or a luirchip hammer ...etc)
then you only need 3-7 units to kill Acheron (really 1 or 2 to do the real fight).
Even better if you have the means to kill the defensive % of the city
and Acheron is dead (plus captured, but leashed on the tile the beastmaster was)
If your uber stack appears then, my 7 unit stack dies. no issue. (no way, even for an über beastmaster, to survive 100+ fights in 1turn, without canibalize)