I've concentrated on that method and it really works - the CPU doesn't cover its chestplate 95% of the times. I only used spells and focused on defending with the shield. I'll put my strategy if anyone would have problems with that game:
1st - None
2nd - Ghost Knight (CPU usually uses offensive spells here, but NOT EVERY TIME - wait till he does that or else you will waste it)
3rd - Magnetic Shield
4th - Lightning Shield (your shield will probably be thrashed during this moment, so if its color is black on the diagram just use Pocket Blacksmith)
5th - Mind thief (to get some more defensive spells from the opponent)
The only small difference is with the Warlock as he will repeat the round with "Let's Try That Again" spell when you deal higher damage to him. He will most likely do that during round 3, so instead of using Magnetic Shield right away it's highly recommended to hit him normal way first, without using anything. After hitting him in the chestplate he will repeat the round and we can use Magnetic Shield then as he can't repeat the round anymore. It should be mentioned that the Alchemist also has a decent card in his sleeve which is freezing your character with Flash grenade. He might use it during the second round, but he also has Long Lance spell. If he uses the latter, remember that he can use Flash granade later on. If that happens then hope for the best (he still hits the shield quite often) or use any of the defensive spells left like Large shield.
Further rounds/additional tips:
It's not recommended to use offensive spells very early on as the CPU will most likely counter them with defensive spells. It's much better to hit opponent's chestplate and then try to steal some of his defensive spells afterwards. You can use offensive spells when CPU has a very small amount of health (around 10%) and he doesn't have any defensive stuff left If CPU hits you anywhere else than the shield and his level is higher than or same as yours it's recommended to quit the battle and repeat it.