If you use the /getdown emote your character starts dancing which seems to be so bewildering to some mobs that they simply can't concentrate and interrupts their casting. Now, this can even work on bosses and boss abilities, although you'd still have to keep aggro which might be a bit of an issue while dancing, but you could always just pop the emote out when on low health or there's a particularly nasty boss ability incoming and similar.
Use it beforeit is fixed and be ware that BIOWARE have recently banned players, just for getting high end gear at lower levels.
basically command /getdown will make bosses casts interrupt.
source: manaflask