I will admit I am thinking from a different perspective, I've moved on from Koth/Skred into a more control type deck with sweepers like Sun/Moon. Skred may be a 1 mana creature removal spell, but to get rid of anything really relevant in the format you need 3-5 Snow-Mountains so it's not really a "1 mana" spell to me and seems really niche.
Chandra's - is nice against a board with 1 creature, but Karn's, with the right support, can create a creature that makes your opponent think twice about attacking (if they have no removal) even with multiple creatures AND he can do it two turns in a row, unlike Chandra who has to wait 2-3 turns to do it again.
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https://www.mtggoldfish.com/articles/instant-deck-tech-goblin-prison-modern