I_N
And thanks to the esteemed I_N, I came up with (I don't know how it will work in practice) a small trick.
What if you summon any elementals (whichever ones you like best:)), and place them against the weavers, and cast Petrify on them? What will happen? Has anyone tried it? If it works, then give the liches the same "gift"!:)
It's still cheaper to cast Portal, summon Devastators, and cast Petrify on them.
Added 2 minutes later
Does that mean that archers can shoot before the portal, but they can't move?