Dread Knight isn't too expensive. After Frostleaf you move on to leveling up your early heroes to lvl 1000. because of the 4x and 10x bonuses they'll surpass the DPS of Frostleaf. You only move on to leveling Dread Knight after you lvl 1000 Duchess of blades
The big problem I have with this game is that the damage you deal is heavily based on RNG. A bit of variation is acceptable, but if the same charge attack has the potential to both 1-hit kill or deal next-to-no damage to the same enemy, then it just makes the game based more on luck than on skill.
The battles became too easy when I can merely stack the damage reduction abilities, then damage buff stack, and clean up whoever survived. The final boss became anti-climactic because he had no counters to this, like every other enemy except for the swordsman demons, that had an attack that damages a % of health regardless of debuffs.