I got my Royal Thorns somewhere in Eastern Islands. It's pretty random - one game I don't think I saw them at all. I wish I had the Thorn Crown though. 3 games and I've never seen that.
I have discussed the merits of Chaos 3 and Destroyer 3 enough, but now I want to explain how I stop the enemy from hitting my troops. For this strategy you need Ice Snake 3, Royal Thorns and Orc Shamans. My other troops are Horsemen, Griffins and Cannoneers, although I don't much like Cannoneers.
Ice Snake is a great spell because it deals massive damage AND slows the enemy using freeze. I spam this virtually every turn (unless using Sacrifice basically) and take out anything dangerous straight away, e.g. Evil Beholders.
Orc Shamans are great because the AI is pretty retarded when it comes to attacking the totems. This is very exploitable. And the Shaman can dish out huge melee damage when required as well. Certainly one of the best units in the game.
Royal Thorns are awesome because of their ability to spawn more Thorns. This is an awesome way to slow the enemy down from reaching your real troops, and then the Royal Thorns are ranged units too.
Horsemen and Griffins are there to pick off any weaklings. The only rage spells I use currently are the occasional Evil Shoal and more importantly Glot's Armor, which basically gives 'one free hit.' But I hardly touch the ragebox otherwise.
Using this strategy, with 97 mana (2 regeneration per turn) I can end most battles within 5 turns, i.e. by the time I would run out of mana. My losses are VERY low and I've got over 600,000 gold now. This is too easy.
Another lame strategy, this one against the Giant Spider. Shamans spam totems, Royal Thorns spam Thorns which spam more Thorns, and pretty soon you can cover the entire grid with your units. Guess what? No more spider drops. I cast Flaming Arrow 3 on the giant spider every turn, because it's only 5 mana and deals over 1000 damage. This is a pretty tedious strategy (took over 20 turns and toward the end nothing could move) but all I lost was 13 Horsemen (they got wedged next to the Giant Spider near the end) and 1 Cannoneer.