The turtle is as easy as in King's Bounty - the Legend. I tried it with all classes on impossible and one and the same tactic works all the time.
The Knight you start with - your tank.
Archmagi - for Magic Shield (in all my games there were archmagi, necromancers and druids in the shaman tower, so I can only assume they are always there)
Demonologists - for damage and summoning
Druids - for damage and summoning
Any unit, preferably ranged - I had beholders, elves, and whatnot available.
The idea is that all of the units have high HP and can take an initial hit from the turtle. It will slow all of them in the first round (guaranteed), which makes things a bit harder. However, there is still a way around it. Put your knight in the middle of your lineup, so that the distance between him and the turtle head is the shortest. Have your Druid summon a bear and be ready to move it as close to the turtle's head as possible, using his run ability.
Now you'll take a mass hit, but that's nothing to worry about thanks to the high HP of your units - as long as it is the only such hit you'll take. What's more important is to move the bears in front of the turtle's mouth, even if this means using the Archmagi's Telekinesis ability to gain an extra hex. The Knight will move 1 square and should be right behind them - if he's not, have the bears defend till he gets there or till they die.
Have the turtle kill the bears and have the knight take their place (use "wait" if necessary). After this has happened, the battle is won, as now things get very simple.
Knights get healing from the hero + magic shield from the archmagi and all they do is defend. As a mage, you can do Magic Spring + Wait + Defend so that you don't lose a charge and gain more mana than you spend - this way you get 10 mana, which is equal to magic spring cost + 1 healing, in essence, gaining a free heal, which frees mana for an occasional flaming arrow. The rest of the troops will slowly drop the turtle's HP and that's it
The question is what do we do afterwards. I have a feeling that a bounty should be next, but doing that without a loss will be quite a challenge. 1c definitely made the game harder this time - well worth the money paid for it, as it will provide us with lots of hours of enjoyment.