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Buff Knight and Drancia are very similar to slayin mechanic wise, both feature pixel graphics and kinda even have the same button placement set up. Both are also on the, I want to say rpg endless runner genre and overall if you want two games that are the closest in my opinion those would be the way to go. Although Drancia is still kinda closer to slayin, which looking at the screen shots you'll kinda see how.
But there's also Punch Quest Which is still the endless rpg runner type but it's not the same set up. It also has pixel graphics and is fun in its own right and if you don't mind it still having a bit of its own mechanics it's still pretty great.