When they died, Sebastian just had to make new undead. If he fixed the ballista, he could also use one of the guys within the city to make a human undead and man the ballista.
Sebastian would be able to hunt for tier-2 fangeds without risking his life. Of course, that meant risking losing the ballista and for sure not earning the truckload of mana he would get by slaying the beast personally, but it was still better than nothing.
Sebastian could almost see it now: his undead returning laden with mana clusters, while he churned out high-quality gear to sell to the denizens of the refuge, earning the clusters he needed.
