The Planeswalkers only arrive when Zehred has no Stormcallers, not from idling. Heroes can be stopped regardless of the computer's level; you just need to switch the hero movement speed. (At 6x speed, you won't be able to stop them, but you don't have to play the whole mission at 1x — just change the speed when the heroes appear). I didn't pay much attention to leveling up my heroes; I used "artmoney" for the whole game. And I also broke the last mission, first the gold, then the Stormcallers. And with 999000, I easily took out Freyda.
Added after 1 minute
I think it would be useful to switch to the structure recommended on the forum.
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The Planeswalkers only arrive when Zehred has no Stormcallers, not from idling. Heroes can be stopped regardless of the computer's level; you just need to switch the hero movement speed. (At 6x speed, you won't be able to stop them, but you don't have to play the whole mission at 1x — just change the speed when the heroes appear). I didn't pay much attention to leveling up my heroes; I used "artmoney" for the whole game. And I also broke the last mission, first the gold, then the Stormcallers. And with 999000, I easily took out Freyda.