I feel like running nissa in this version at all might be a mistake. The biggest benefit to dropping her is negated by the lack of a mana outlet to turn her passive into a win condition. Creating 3/3 elementals is alright, but I'm skeptical about the value of that compared to making the deck go faster with another shock or more baby Chandra.
Big Chandra is also a bit of a flex point in this list. I really like her against the right meta, however without more ramp than reef alone, she tends to come down too slowly against everything except slow control decks. Consequently I'd probably move at least one of her to the sideboard, maybe two if your meta doesn't really involve a lot of nexus.
Instead any 2 or 3 of can be bumped up instead, probably a bit dependent on what you expect in the meta.
4 baby Chandra 4 shock is probably the first thing I'd change though since they synergize with each other, and baby Chandra gains a ton of value when you're running 8 burn spells and 12 creatures that combo with her.
One is an Elemental-heavy build that's primarily RG where you splash U essentially just for Risen Reef and Omnath, Locus of the Roil. Here you want maximize the tribal synergy: you play 4 Creeping Trailblazer and 3-4 Chandra, Acolyte of Flame for big blowout turns with Risen Reef out, 2-5 between Living Twister and Overgrowth Elemental to stall aggro, and some combination of Shock, Lightning Strike, and Lava Coil to clear critters in the early game and burn the opponent later. Chandra, Awakened Inferno is rather clunky here, so it might just be a 1-2 of in the SB against slower matchups. Nissa, Who Shakes the World is also merely ok here. She's just such a solid card that playing her is never really wrong, but this feels like a suboptimal shell. You're not really ramping into anything ridiculous but more curving out and turning creatures sideways, and 4/3 Elementals with Creeping Trailblazer out are only marginally better than 3/3s.
I think this version can be most successful in an aggro meta filled with WW, RDW, Vampires, and in general decks where you 1-for-1 their best creatures and win by CA with Risen Reef and Omnath, Locus of the Roil triggers. It does struggle a lot against control and combo decks because you can't apply enough pressure and eventually either run out of resources or get comboed before closing out. The biggest problem is that you aren't really doing anything crazy and have to fall back on the beatdown plan, but the deck lacks a proactive T1 play unless you start playing bad 1 mana Elementals which aren't aggressive enough and are also horrible topdecks without a Risen Reef out. The best aggressive line is something like T1 Shock their 1 drop, T2 Creeping Trailblazer, T3 Chandra, Acolyte of Flame and swing for 6. Hardly impressive, and there aren't really decks out there which both have a 1 drop and will have no blockers on T3 so you'll mostly be just sending out the two tokens. Against creatures-based decks it's better to T2 Leafkin Druid into T3 Omnath, Locus of the Roil'ing their 2-drop, and even here we aren't exactly rocking.