NerVNotDeaD
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this ability is only given to Warlocks, i.e., heroes from the Dungeon faction; the spell deals 50% more damage and consumes twice as much mana.
Added 10 minutes later
regarding Goblin Shamans, it all depends on the ratio of the number of goblins to the sum of the hero's level plus the spell level. For comparison, if a level 25 hero casts Shock Earth (a level 5 spell), the spell needs to have a 25% chance of not working, which requires 725 goblins in the squad. Consequently, the more goblins there are, the higher the probability of interrupting the spell.
I think if your archers damage them well with the first shot, you can cast spells more freely afterward.
Added 10 minutes later
regarding Goblin Shamans, it all depends on the ratio of the number of goblins to the sum of the hero's level plus the spell level. For comparison, if a level 25 hero casts Shock Earth (a level 5 spell), the spell needs to have a 25% chance of not working, which requires 725 goblins in the squad. Consequently, the more goblins there are, the higher the probability of interrupting the spell.
I think if your archers damage them well with the first shot, you can cast spells more freely afterward.