The whole internet is wrong on the level of this synth. It is NOT level 50 clothcraft. I have level 50+ all crafts, including 51 clothcraft, and I failed on this synth 5 times in a row. Thus, the information about this synth being 50 clothcraft is ********* Period.
I just finally was able to synth one of these after quite a few fails. The level 50 is wrong. I am 53 and got a skill up on it. I also had advanced synth support when i did it.
I do not have the core robe for this, so i'm unable to test it, though the first recipe i'd try if i attempt on it would be the Royal Squire's robe in an earth crystal, together with 1 velvet cloth, and 1 silver thread, and if that doesn't work, a wool cloth instead of a velvet one, if that doesn't work, then linen thread and wool cloth, and then linen thread and linen cloth. If neither works, trying yourself with different types would be more effecient than my guessing.^^