I haven't found that too reliable either. I think the set is inherently flawed because you need to damage something(three times!) in order to gain some much needed DR. Teleporting through some packs of ranged or leapers can be really painful.
What fire spells have you been using? Disintegrate and Energy Twister seem like easy picks but I can't settle on the third slot. I'm thinking Mammoth Hydra is the best choice because the game treats the "you can only have one up" as a cooldown so you get max twister spam from Etched Sigil. But you can cast Hydra yourself to apply three spells reliably.
I've tried Blizzard(seems like a waste with Etched Sigil), Blackhole(kinda nice), and Magic Weapon(the ignite rune doesn't seem to proc often enough).
Manald Heal + Tal's + No archon builds seem viable, I'm going to shift towards those.
Leadsalad
I tried out Mammoth Hydra, I suggest you do the same. It works even better than I expected. Etched Sigil respects any cast made by you or it. So, if you cast a hydra before spamming disintegrate, you'll ensure everything that procs from ES for the next ~14s is a twister. Group everything up on your hydra and they'll have three burns easy.
