User blog comment:Miskos3/Fairy Tail Episode 253: Tartaros arc: Silver Memories, Review/@comment-24039234-20151003120305/@comment-5303512-20151004135450

Well it doesn't really matter if the element of the slayer is the same as their intended target as long as it works on them. Look at the Dragons during the Eclipse Incident. Even though the Dragon Slayers were weakened they could still hurt the Dragons.

Now For Silvers whole plan, I think that was the point, he thought he new he had a plan because he had to be morally "Grey" (pun intended) but realizing that his son was still alive that everything he did was pointless. Just like Itachi in Naruto.