I've got it, and I think I've only uncovered a bit over half the map.

Well its random, but mostly it is on the end.

Demonologist life drain works.

The only way to resurrect plants is with sacrifice spell or the drain life demonologist hability. U can sacrifice inquisitors, or another level 2-3 troop and u gain 2-3 royal thorns. Then u resurrect the sacrificed troop. If u seen u are going to lose more r thorns than the ammount of gained with sacrifice, cast invisibility spell on em. And remember r thorns are level 4 units, u can cast fear on any enemy level 1-4 (if u have level 3 fear or 1-3 level 2) and worry only about level 4-5 enemy stacks. If u can, use demonologists. Demonologists + sacrifice spell, too hard u lose any royal thorn.

Time back is a common spell to get in dersu. You will usually find armageddon, time back, or death star in dersu but death star is usually found in reha.

Doesn't Time back require you to move the royal thorn first?

no it doesn't. The only way that time back will not work is if an enemy is occupying the spot where you were at the beggining of the previous round.

Are you sure? I seem to remember not being able to timeback a stack of Red Dragons once because they hadn't moved since the last round (I just had them fire off a flame breath first turn)... I was a little irritated since my game had a paltry selection of Reds. :p

Yes, I remember the same happened to me.

No. If a unit hasnt moved yet, time back doesnt work on that unit. Time backs move a unit to the initial position of the turn, so if it has not moved yet, its no sense time back will work.