"Do not be afraid of sudden fear nor of the onslaught of the wicked when it comes"
This is the Hebrew word yare which means fear, afraid, even reverence. Do not build your life around the possibility of something bad happening to you. Don't rehearse all the bad things that could happen to you.
You have a choice whether you will let your soul be gripped by the emotion of fear. You don't have to let this happen.
This is the Hebrew word peta which means suddenness. It has the idea of quickly, unexpectedly, impulsive. Solomon is saying that if you follow the path of wisdom, then you do not have to be afraid of sudden fears. You will be ready for these. You will have seen the signs coming.
He is saying that you should not give into a paranoid point of view. Some Christians have developed such a negative evil-is-everywhere mindset that they are skittish and pessimistic. Don't let this kind of thinking grip you. If you are following wisdom, God will protect you.
This is the Hebrew word pahad which means dread, fear, terror. The idea is that some thought rises up in your mind and begins to consume your thinking – some terrible thing that could happen to you. Don't give into this intruder in your mind. Don't give over your thought processes to an emotion. Push back on the emotion and express confidence in the Lord.
For some this sudden fear is rooted in reality; for others it is bizarre. It could be an IRS audit; it could be a past sin coming out; it could be an unlikely attack; it could be a series of improbable events. The point is that these are not events that are happening but are, instead, things that "could" happen that are invented by your mind that seek to pull you into a pool of fear.
When one of my daughters was little, she saw a TV show about a fire in a house. We had to spend a lot of time praying and reassuring her that our house would not burn down. She had allowed herself to be completely gripped by a fear that was not going to take place.
I watch as grown-ups do the same things and change their whole lives to try and avoid an imaginary fear. Don't do this; don't let fear dictate what you do or how your live your life.
Notice that this verse is in the midst of a section about what following wisdom will do for you. But Solomon still needs to tell us not to let these imaginary fears take over our thinking.
nor of the onslaught of the wicked when it comes
This is the Hebrew word shoah which means devastation, ruin, storm, waste. The idea is clearly that the wicked will try and bring their destruction upon the society and the righteous, but those who are following wisdom do not have to give into the fear that this will destroy them. It may destroy your worldly possessions, but you don't have to give into fear over the thought of the wicked succeeding.
Solomon is talking, in this proverb, about two kinds of fearful things: one, a sudden fear that pops into your head; and two, the agenda of the wicked to get what they want. Solomon's advice is that you should not allow either of these kinds of fears to grip your mind.