Avraham Abulafia [the mystic from the Middle Ages I have mentioned before] had an approach צירוף אותיות combining the letters of the Divine names of God. His approach is brought at length in the Remak (Moshe of Cordoba) and Rav Chaim Vital.

There was a  debate because of his books of prophecy. That I imagine was too much to swallow for many people. But even the Chida (Rav Chaim David Azulai) brings him in Shem HaGedolim.  

[One thing I noticed was his sympathetic approach to Jesus but from what I can tell that was not the reason the Rashba was against him.]