Recommendations for Slate Tiling Concrete Step

I have an outside concrete step (approx 4 or 5 steps) where the steps are not very level. They are about 6 feet wide and are out by about +- 3/8 inch. It is pretty noticable when one step is tilted one way, and the next step is tilted the other way. Everything is solid, it is just that the builder was an idiot. Instead of jackhammering a ton of concrete, I was thinking of covering the works in slate tile.
1. To get everything perfectly level, can it be done with the thinset/mastic alone? Or would it be necessary to build up first with some sort of concrete (remember, I am talking about approx 3/8" over 6 feet).
2. What type if thinset is used for outside that will stand up to cold temps.
3. Any other tips on adhering the tiles to make it last?
Thanks in advance.
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grpman wrote:

I'd level it some first, doesn't have to be perfect. Reason is that I like my thinset "soupy" enough so that the weight of the tile will depress the combed thinset (after/on the leveling base) enough to spread it under all the tile.

Any
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