Notice in the .gif that I roll away from the spell after taking damage. Had I stayed still, I would have taken damage again and again as long as the spell persisted.
But rolling moves me out of the spell and since the monster does not turn to face me (spellcasters cannot move at all when casting), I avoid further damage. You can also just run out of a spell, but rolling is faster.
The gif is a little small, so I took a couple of screenshots to illustrate how it works. You can also see a larger version of the .gif on my Twitter feed.
(https://twitter.com/SuperSixStudios)
Goblin Spellcaster charges up his spell indicated by his animation and the particle on his hand. You have about two seconds to react once he starts doing this. Note also the Goblin Spellcaster's shirt is yellow and the melee Goblin's shirt is blue.
Despite blocking the melee attack from the Goblin Grunt, I still take damage from the spell since spells cannot be blocked. This is indicated by a yellow number rather than the red numbers used for melee damage. The spell persists for about 4 seconds and if I remain in it, I will continue to take damage.
In addition to the animations and particles, there will also be sound effects to let you know when a spellcaster is charging up a spell and when it is cast.