I've added level 3 spells for dueling heroes. I'm particularly highlighting Raise Dead, which for 14 mana resurrects 330 HP, i.e., significantly more than Chaos (Lightning for 10 mana deals only 160 damage). I've removed the HP penalty for raising undead. Thus, Raise Dead is used as a counter to targeted Chaos spells. The second key point is the addition of Remove Curse, which costs only 5 mana and thus cheaply counters enchantments. The third key point is the addition of the Ring of Cold, which for 15 mana deals 144 damage (a very strong effect; for comparison, the budget Fireball deals 100 damage for 9 mana).
Among other added level 3 spells: plague for 11 mana (90 damage per turn, 10 rounds), slow for 9 (reduces initiative to 60%), punishing strike for 13 (+10 to attack), wall of swords for 10 (168 HP for the area), lightning for 10 (160 damage).

I adjusted the damage from the Secret Crystal, reducing the damage from 180 to 120 (it was overpowered before). I increased the damage from the Swarm of Wasps from 80 to 100 (this is because a knockback of 0.6 doesn't push into negative values).

I reduced the mana for dueling heroes from 50 to 30 (because in real conditions, with 10 knowledge, they would hardly have a full 50 mana), which significantly affected the magical tactics (the cost became more noticeable).

I assigned the creatures their actual strength levels instead of formal ones. The Hydra is now the same level 7 creature as the Black Dragon. Accordingly, a hero attack now deals only 32% damage to the Hydra (it was 50%). Now the Hydra is much more dangerous because it doesn't die from 2 hero attacks