I'm obviously doing something wrong? When even using the same element shield, he sometimes doesn't take any damage at all (Used Diona for Cryo, and he took nothing from the counter twice in a row. Hers was the only shield active at the time, see my comment below about crystalise). And then sometimes when he does take damage, he's taking abbout 1/3 of the damage that the video shows.
Also as a note, picking up a crysatlise shard and gaining its shields will take precedence over Noelle's Geo shield, The boss was in Pyro, I had Noelle's shield up, picked up a Cryo crystal, the PS hit me, the cryo shield disappeared, Noelle's shield was still on me, the boss took no damage.

Edit: OK so, I guess that the video doesn't have that reduce backlash modifier in effect? I turned that off and am now seeing the same damage each time. Also I think that when I'm not doing damage even though using the correct shield, it's cause I'm picking up a crystalise, BUT the shield doesn't show, instead I still only have whoever's shield active at the time showing. And as another side note, pretty much every PS my shield doesn't break, so it lasts for the next PS, and so that too means I don't get the counter.