Sling is awesome. This is a list that's kind of a mashup of other Sling decks running around, with added consistency/board control options to optimize it for gauntlet.
It plays like the FTP Howling Peak decks of old, except Sling makes it a little faster. Instead of a landing a big boy turn 5 and Peak turn 6 to Gun Down, then Mirror Image turn 7 and taking over from there, you play sling turn 3-4 and a big boy turn 5, kill something and take over from there. It's a full turn or two faster than Peak at almost the same level of efficacy for its most common line of play.
In gauntlet, Sling can only really lose to aggro and power/influence screw. Desert Alchemist helps with both. From Elements helps early and can be a beater later. Market Hailstorm protects us from Tokens and Onis and the like. Petition and Beseech ensure we hit our power and influence. Ramp is kept to a minimum to avoid fragility. I hate getting 2-for-1'd by Vara's Favor every other game. Permafrost aggressively.
Ila & Mizo is the best Slinger I think. Touvon and Spitflame can both remove multiple threats on summon. Cirso is as devastating in gauntlet as always. Ixtol is meh, he can be replaced, but I like his secondary ability. He functions here basically as a Worldbearer Behemoth with a lower influence requirement, allowing Touvon to be our only greedy TTT option.
As always with a midrange gauntlet deck, always be wary of Harsh Rule.