Question

which fallacies is used to undermine someone

Answer 1

Ad Hominem-This is the fallacy of attacking an argument's author, rather than the positions they hold. In American politics, it is probably the most common fallacy, and it is certainly the most pernicious. For instance, in 2008, when a woman shouted out to the candidate at a John McCain campaign event in Minnesota that Obama is an Arab, that was an ad hominem attack. She meant it as an insult to the person of Obama, attacking him rather than the positions for which he had stood.

Hasty generalization- This is the fallacy of jumping to conclusions without sufficient evidence. It is a matter of taking one or two instances of something and turning them into a trend. For example, if – just based on Rachel Dolezal – we argued that white people across the United States are trying to blur racial lines, or that they are regularly posing as black to gain some perceived advantage, that would be a hasty generalization fallacy.

Slippery slope- This is the fallacy of assuming that we are inextricably tied to that path simply because we have taken one step down a given path. It's the alarmist fallacy we see almost inevitably when a TV talk head tells us we're doomed (doomed!). It is the fallacy, for example, that prompted some leftists to believe in 2001 that the United States had inevitably boarded a sluggish train to communism only because the USA PATRIOT Act curtailed certain civil liberties in some circumstances. The truth is that there is a broad gap between only expanding surveillance systems and totalitarian rule of the mid-twentieth century era.