I like some of the already existing card choices you have here, especially Al'Akir and Bog Slosher. I think Bog Slosher should probably go up a tier, though. Soldier of Fortune, however, I'd say is like a Tier 0. It's absolutely not worth having a stat stick early on with that big of a drawback. As for your custom cards, there are a few problems, discussed below.
You have a few themes, Windfury, start of turn, selling Elementals, and Flame Elemental. Out of these four:
- Windfury is good, but not enough support. Out of all these tactics, this is probably the one I'd like to see added in to the Battlegrounds. It can scale very well into mid and late game, especially with Divine Shield and it has been proven to be effective already with Murlocs.
- Start of turn, too gimmicky and niche. Start of turn does not scale well at all, and your 6 mana 5/5 that double start of turn effects will only affect perhaps 6 cards (including three Hero Powers, I do not know if you took that into consideration). So, only 9 cards in the entire Battlegrounds pool, and none of these buffs scale. That's awful. This kind of card is like a Brann/Rivendare, but is a whole lot worse *and* it's in Tier 6 unlike them, both in Tier 5.
- Selling Elementals, far too weak. Look at Malygos. He procs off any Battlecry. You probably need to rework the tiering of these two and make them far easier to activate, or have a much bigger payout.
- Flame Elemental, very hard to judge. The only thing we have to help balance are Primalfin and Muruzond. I hesitate to add an easy way to give triples.
If I were you, I'd look at a few cards such as Cauldron Elemental (Tier 4 or 5), Whirlwind Tempest (Tier 6, the Megasaur of Elementals), Siamat (Tier 5), Mountain Giant could be *very* *very* interesting, as it rewards player for floating cards in their hand, something that not everyone can do all the time. No idea what tier Mountain Giant could be, though in honesty, the idea of it in battlegrounds is a little far-fetched.