Chain will kill 15 priests (and even then, a deviation might occur), while dispel will remove the entire stack from combat.
Added after 1 minute
Maybe after the first mass confusion you should have cast mass slow as well? It would have given the hero more time to finish off the priests. Even if not completely, there probably would have been fewer losses anyway)
the imps are dead meat either way—4 hundred rogatics were saved regardless, as well as 2 hundred cerberi—I fed them to the 2nd hero)) And +- five cave trolls won't make a difference, imho.
On slow, deviations can also occur, and mana is limited—I didn't want to waste a turn eating.
2 dispels had to be cast anyway—no matter how you look at it.