Need help: Act 3 Cruel Piety in the Crematorium

Dear fellow Templars,
I run a level 49 spellcaster templar with minions for tanking, single spork totem for base damage and firestorm for burst/tageted damage to tough mobs. For boss battles I use my spork totem (shielded by skeletons if needed) as the main damage source, as it is nearly impossible to stand still for prolonged time needed to use the firestorm. This way I soloed Cruel Vaal yesterday and today I got to the first Act 3 encounter with Piety who has energy shield. My skeletons die too fast to her lightining attacks, and then the totem dies as well. So by the time I recast them, she already starts to regenerate her ES. I don't have problems surviving with 75% lightning resist, I just can't do any damage :(
Any advice on how to beat her?