Yeah, the turtle is no big deal, I think I could beat him 1 Archmage and 2 Griffins if necessary.
Turn 1, move your Griffins forward 4 squares, separate, then move the new group into melee range and defend. Cast Stoneskin on the Griffins. Archmage puts Magic Shield on the Griffins. Turtle deals about 40 damage and pushes them back 1 square.
Turn 2, move your Stoneskin Griffins back into melee range and defend. Cast Magic Spring on the Griffins. Turtle deals about 25-30 damage and pushes them back one square. You gain 5 mana.
Turn 3, move your Griffins back into melee and defend. Cast Healing on them. Turtle deals about 25-30 damage and pushes them back one square. You gain 5 mana.
Repeat turns 1 - 3 until the turtle is dead.
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I've cleared about half of Freedom Islands now, the Orc Embassy looks like it's going to be very tough to beat. I have replaced my Archmages with Beholders and found 4 more Royal Thorns. No change in strategy yet.