For those after the achievements, it appears to be random which two named will spawn in a pair. kill the pair in the alpha order and if possible, within one minute of each other. The Tumbling Boulder spawns during the time in-between the first named death in the pair, and the second named death in the pair. Avoid it (even on the platforms). healers can heal from below, and do not need a sigil.
Sigil to Named reference list:
Sigil of the Soaring Heart - color blue SE Alabaster Moonbreeze
Sigil of the Golden Flame - color red NW Sunbird of the Dawn
Sigil of the Darkening Depths - color purple SW Grinning Monsoon
Sigil of the Hardening Heart - color orange NE Diamond Earthshaker
If done right, can get all 3 achievements of this trial in one run.