I assume reduce mana by 20% or 25% is of the enemies current mana production. Lets say average of 10 tiles for 100% is 10% a tile. So a hero hit 8x should have 80% mana. 20% would be 16% leaving the hero with 64%. So instead of 2 more tiles for for 100% you would need 4x more hits to reach 100%. Lets go fast at 7 tiles for 14.285% each. Lets say you hit 6x and the hero has 85.71% mana. 20% is 17.142% leaving the hero at 68.568%. 2x tiles should put the hero at 97.138% which means should takes 3 tiles to reach 100%. All this is under the theory of no mana boosts from buffs or troops. If we throw in these modifiers you can basically reduce all the tiles need by 1 or 2x easily. So now you have casted your skill and need 7, 10 or even 12 tiles to regenerate your mana reducing hero and you have actually put very little hurt on the opposition. So now the bane of our existence. NINJAS! and these newer heroes that build mana tiers. I know mana speed says charge, but in ninja tower they take all sort of hits before charging. Floor 4 took 10hits per tier. Floor 5 took 12 hits per tier. Either way. Mana reduced vs a ninja is like trying to sneeze to put out a fire. Check following pics of Chao vs ninja.

Now checkout Chao vs ninja with Guinevere followup

Anyway… looks like mana is only reducing vs a single tier. Very disappointing. Feel like mana reduction needs be reworked…