The troops ignoring the enemy was also something they did in the first itteration.
I'm currently on 6-5 on hard. Half of the legendaries are pure trash as they focus on crit damage... something which is almost useless without a high crit chance. A crit focused character will be around 20% crit chance. A +20% crit damage increase will amount to 4% extra damage. Some of the commons can outperform that depending on the character. A straight +5 damage to the lvl 10 footman's 20'ish damage will be around +25% extra damage.
The enemies were made harder in the previous patch. Especially on hard mode. One of the best units is still the auxiliary despite being nerfed. The assassin is a waste of space. I gave him the chance of the day and got him to lvl 10. Here's what happens: He triggers his special. Spends ages charging the special and before he can get it off—he dies. The times he does get it off he'll have wasted an immense amount of dps on the charge time alone. They should make the special take 1/3 of the time it does or AT LEAST half. It's really bad as is.
I struggled on hard too, then I spammed auxiliary again and beat everyone
The best powers in my experience is still the full defense.
Having the enemy lose 70% of the damage with exhaustion (frost) is indeed okay. But what usually happens is that the front lines still dies quite fast, when they are at the enemy's castle. As the enemy units will have clumped up. They will then proceed to tear through the rest of the lines by their sheer numbers. The 4 seconds prevents the front line from falling and it's usually enough for the castle to fall. The total destruction at max level is also really good. Until the very end of the game it can one–shot most enemies and it'll severely hurt their champions.
Cheese the 6th zone with the castle on the later levels. Send a single very defensively equiped footsoldier at the right time. So that he fights under the castle fire. The enemy will die quite fast and you'll slowly turn the tide against them. Once they send a large force use the total destruction spell and they all die or at least they are softened up a lot. Repeat this a few times and the enemy will run out of steam in due time. Again the auxiliary is your main damage dealer, give him befitting equipment.