The Best Offense is a Good Defense
I generally avoid talking about specific game systems, but recently I started a D&D 5E game and I have some experience that can help you in building characters. In one short game, I had our monk character go up against a friendly monk NPC in a fighting match. During the fight the PC had an ability to cause disadvantage and no matter what I tried, I could barely hit the PC. This allowed the PC to overcome the superior NPC―and could allow you to do the same. First, let's do a quick review. D&D 5E is designed to be a simplified game, and the main mechanic it introduced to do this was advantages and disadvantages, which are meant to reduce the number of die modifiers. The latter is a positive modifier while the former is negative. One obtains these modifiers through skills, abilities, feats, situations, inspiration and DM fiat. The mechanics themselves are simple: both advantages and disadvantages grant you a second roll. With an advantage you take the higher of the two, and with a disadvantage you take the lower. But there are two sub-rules that are even more powerful. Advantages and disadvantages do not stack, and they cancel each other out.
Most people seem to love advantage as it gives you that higher roll, but as you know by now, I'm a fan of disadvantage―just as I'm a fan of high constitution and AC. These are things that keep you in the fight, hopefully longer than your foes. I haven’t done the math, but that disadvantage roll significantly reduces your opponent's chance of success. Secondly (and perhaps more importantly), it cancels out any advantage roll your opponent might have, thereby reducing their capabilities. This is huge in any D&D conflict. So get your disadvantage on. Get into the breach!