Empty Slot is for [Magnetic Rush]
Evolutions
Evo 2: Whirling Flurry
Evo 3: Echoes of [Rage]
Evo 4: Swift Transmute
Evo 5: Chain Shatter
Chain Shatter with a grouping tool like Melee Vortex puts in crazy work
Can do Blast instead of Gas but I don't like it as the enemies can get flung way out of the way of where you want them to be.
For a higher tier build use [Galvanized Elementalist] with [Primed Fever Strike] and [Molten Impact]. It makes the shards do more damage. Not sure about the Gas damage as I don't have [Galvanized Elementalist] or PFS so I can't test it myself.
For yet another boring and generic influence build swap out [Magnetic Rush] for [Voltaic Strike] and Melee Vortex for Melee Influence. I specifically didn't build for influence though because everyone builds for it and I wanted to try something new and fun.
I like running it with [Nova] for more explosions per enemy.