is this channeled? or cast time.. maybe its something that HAS to be interrupted.. I haven't gotten there yet but when I do I'll make sure to watch out for this.
It can't be avoided. It is channeled but the first tick does 70k damage, tank gets one shot then he one shots everyone else. The first tick is INSTANT there is no time at all to interrupt, even if you had the fastest reaction time any human has ever had in history you could not interrupt it in time.