There's no need to use triggers just to get the alternate transformations. This can be done easily through the Object Editor by having at least two versions of the Gnome Arcanist, one for the normal form, one for the alternate form that you want.
View attachment 419170
Notice that the field "Required Animation Names" has been modified to "gold". That means that the Hero Arcanist in practice has transformed into his water form, as it goes. This is because all relevant animations that the model of the Hero Arcanist has must include the tag that was initially specified (Required Animation Names).
The same thing can be done with the Hero Arcanist's other alternate form, by changing "gold" to "lumber". I've also taken the liberty of uploading a test map to demonstrate the Gnome Alchemist's transformation into its two elemental forms.
Alternatively, you can manually add the animation tag through the GUI Trigger Editor. The function which you're looking for is this:
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Animation - (Add / Remove) the (insert animation tag here) animation tag to (Gnome Alchemist)
When triggering a transformation, you will need to listen for the moment a Gnome Alchemist casts a dummy spell, keep track of the moment when said Gnome Alchemist transforms back to normal (or dies), and add or remove the animation tag accordingly. Unless the transformation comes with some advanced trigger effects, it would be quite the hassle to implement it as closely to game mechanics as possible.