When fighting spell casters, and no one in your group is dedicated to interrupting, or the interrupter is busy, keep the Spear-Shield mastery empty so that if a mob starts an induction, you can drop your current gambit, pop Spear-Shield, give them The Boot, then carry on.
When you're tanking a spell caster like a Defiler, figure our their induction patterns. They're not likely to begin inducting another spell straight after you've interrupted one, so you can use a few aggro gambits. If it's been a while since they tried an induction, start building a Spear-Shield-Spear-Shield in anticipation.