Well, first action from any mage on the ambushing side is "dimensional anchor". Unless you want to let them a chance to escape, but even in that case, it's better to let them find a more dangerous way (like flying or running).
The ambushing side don't have any caster able to do so ? It's not well prepared enough.
That or use the ol'good "write on paper what I want to say".
There were a bunch of way for Tarquin to give the drow that order. I doubt him knowing drow sign langage will end up being as critical as most people seem to think.
Oh, and I love how Tarquin determines Nale's consciousness based on his love of his own voice.