I'm unsure how it works on bosses (I've only seen it used on Shadow mobs), but Shield Breaker + Quad/Tetra Punch(?) can do some decent damage. Other than that; the only skill that comes to mind is Shard of the Glacier.
The reduction to electricity resistance only lasts for 3 seconds, so it may or may not be that significant, because Guardians aren't that reliable. Also; you probably already know this, but be sure to have around 320 LK for the 60th floor boss and 400 for the 72nd floor boss.