I was unsure if I should go for Theresc or not, eventually I decided not to pull shards for her.
Reason number one is that her signature move A3 has only 75% chance, if booked. In all cases I would like to use Theresc it would be crucial that she consistantly applies her decrease resistance debuff. Therefore, it is a necessity that the chance is 100% if booked otherwise it is not realiable.
The 75% chance reminds me on Ultimate Galeks HP burn abilities. At the time when the Doom Tower was introduced I trained Ultimate Galek because of his HP burn skills. He was the only option for me at that time. I used him for the first couple of months, but I decided quickly that I have to replace him. His HP burn abilities are not realiable enough for the Frost Spider. Now, I do not use him anymore as HP burn champion. I tried to use him for Hydra but without any success.
Due to my past experience with 75% chance debuff champions (Ultimate Galek was not only one) I do know that those champions are not realiable enough. In hindsight leveling Ultimate Galek was not justified taken into account all the ressources I put into him and how often I did use him afterwards as HP burn champion. I am pretty sure the same will apply to Theresc.