the weave serpent shield states that all damage is "reduced by 1" not "reduced to 1", So if you roll a 6 for a D6 damage weapon, the serpent would take 5 wounds.
Straight numbers game, it will take 12 dark lance shots to kill a wave serpent. you hit on a 3+ (.667) and wound on a 3+ (S8 vs T7, so .667) with no saves. The Average on a d6 is 3.5, but the shield reduces it to 2.5. After you enter all of that, it comes out to roughly 12 dark lance shots.
Again though we all know that straight math is not king in this game, but nowhere near should it take 30 shots to kill 1 wave serpent
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