royal snakes are on debir in the house with a guy asking you to clear his basement. After you do it, he will sell snakes. Beware thou, there are lot of snakes in the basement
I'd go kitting a bit before taking the quest, if you want no losses there.
also, divide inquisitors into two stacks so you can ressurect them in the end and cast that holly anger more times to get rage for your pet. Use stoneskin A LOT, get rid of enemy archers first and bla bla. if you have higher level units than enemy archers, you can cast fear on them (if you got it early in the game) otherwise try to burn them asap, especially plants.