I dont have any problems applying Shard of Glacier Debuff on bosses except the 10% fixed miss rate, but i still think its 100% Eletric debuff, the math itself is just complicated.
If the mob has near 0% eletric resist = -100% debuff = 2x normal damage.... (you hit normally 10k with electro, then 20k debuffed)
If the mob has like 70% eletric resist = -70% electric dmg +100 electric dmg % = +30% eletric damage -plus MD- (you hit 3k with electro (7k resist), then 13k debuffed (3k (normal damage) +7k (resisted before) +3k (30% bonus dmg of the Shard glacier)
That explains why sometimes seens to be like 800000000x normal damage.
For the case of Shard of Glacier missing: Do you have enough luck? Do you miss other skills often?
PS: On Chaos Bosses please forget about your electroshock passive when doing electroattack. This will NEVER work on bosses x_x.
The bosses seen to be resistant to mana web, rust etc... (Hard to land), but electroshock is like 100% resist or 99,99999%
EDIT: The mechanism seens to be the same for the area debuffs. Like Undine's garden. The description says it lowers the resistance of the mob by 17% (master). In mobs with low resist-base, you will see this 17%; however, when fighting against bosses this 17% will turn into 50%... 100%... easily.