Something to remember (or so I seem to remember

), is that things with second shape will not change in to that shape if they have more than 100 fatigue. This means that a skratti witch have tired itself out by casting spells will not become a werewolf. This is also why stellar cascade is a popular way of killing Chayots. If an unit does not have 100+ fatigue and have a second shape then it is impossible to kill it with just one attack. Even if hit with something like soulslay which cause 999 points of damage it will still have at least 1 hp left in its second shape. More if its a Skratti because they regenerate.
Unfortunately I think the Skrattis first shape is the wolf form and its last shape is the werewolf form so this is harder to abuse than you'd think.