I think the best answers for Mentor depend on the strategy. If you are trying to combo out and you are using black then Dread of Night is one of the better answers because of the low casting cost. This in an Oath deck though, so it is a benefit to keep the Mentor in play. Toxic Deluge is good if you are playing storm or something with Death's Shadow, but shadow is not the best with all of the Swords to Plowshares being played right now. If you are on red, Lightning Bolt Is good if you use it as a response to the prowess trigger, or when they are low on cards, or a combination of Engineered Explosives and Sudden Shock. Swords to plowshares is definitely effective, but if you are worried about the tokens then it is best to not let it resolve in the first place if it can be avoided. I don't think many Gush decks use Mana Drain, so very efficient play and sequencing would be required to win. However, that is not a very predictable strategy because of the complexity of playing Gush optimally. Tinker and Sphinx of the Steel Wind are one of the best options, but Gush Decks are normally light on artifacts. Taking these things into consideration Balance and Supreme Verdict seem effective for controlling the board state. Balance seems more enticing using Young Pyromancer because it doesn't require as many lands to operate.
Considering all of these factors I think that most decks effective answers for Mentor except the Mentor deck itself. This presents me with an interesting opportunity to test some of these theories. Young Pyromancer strategies are not going to be simple to play considering the prevalence of Oath of Druids decks at the moment. It looks like I've found my task for theday!