Dazzle and Ice bolt to protect against shen-ra speaking.
I think a transpose market is key when you are building around a primal relic.
I don't like running peak with Rost because it wants fire sigils in the deck - so I like mirror better as it does what peak does but better.
I've been seeing a bunch of sling decks running tons of 6 attack units, but I don't think that's the right way to play sling. You only need to remove one or two threats to take over the game. Sling is a draw engine at its core. Any x/6+ becomes a cantrip, so campfire and tota colony seem like the best cheap options.
4 primal sigils is a bit low for my taste too, I could see just going up a bit on power. You need sigils so you can bolt your Rost to recur the sling shot and stop silence/stun.
I plan on testing alot more once I hit Masters.