This should be self-explanatory but here goes; you get Judgement Points by battling in the Latest Event.
There are 3 tiers to compete in, the first gives 10 Judgement points for each successful match, the second tier gives 15 points for a win.
You will need to play in the 1st to unlock the 2nd and play in the 2nd to unlock the 3rd and so on.
Depending on your skill level you can play and win 10 times in tier 1 to get this achievement or you can try the second tier (once unlocked) for a faster way to get to 100.
Mind you, each game requires to get a NET score of 20 wins to complete a match. Veterans or those with the right set-ups will breeze through this no time. Have fun.