OK! I did a lot of reading/research and finally figured out a fairly easy path to this achievement. Once I got my strategy and character figured out, it only took about 20-30 minutes of playing to rack up my twenty 'Just Impacts'.
1. Play as Yoda. Because Yoda can't be thrown and is immune to high attacks because he's short and these type of attacks miss him.
2. Enable the skill Master Impact (which requires a lvl 8 Yoda) in the character creation menu. You'll also need an accessory of +80 Impact or higher and the light saber of +50 Impact. Since the Master Impact skill requires you to have an Impact total of at least 130. Master Impact turns all normal Guard Impacts into Just Impacts but makes the timing of the Guard Impacts a little bit more difficult but not nearly as difficult as successfully completing a Just Impact without this skill.
3. Play in Story Mode or Tower Mode, not Arcade Mode.
4. I got most of my Just Impacts against Raphael on Normal difficulty. Which is the first character you face in Yoda's story mode.
5. The KEY to performing a Just Impact is to simply *press forward* towards your opponent while also pressing Guard (A button by default) at the *same time* just as the attacker *starts* his/her attack. I guarded against mid level attacks and ignored trying to block low attacks for simplicity. But you can try to block low and mid if you are good at predicting the level of attack. Both mid or low blocks will generate a Just Impact if timed correctly. This works especially well against attacks from Raphael at a mid distance away from him because his attack at that distance is easier to time.
6. You can observe a successful Just Impact when it occurs by observing a large *flash of red light* at the point of impact and a sound effect (a Guard Impact is the same flash of light but green in color). It's obvious when it happens when compared to a normal block or hit. After a little bit of practice, I was able to get a couple or more Just Impacts per fight and hit 20 in a short amount of time.