Yakuza: Like a Dragon

Based on my playthrough, I was able to get this achievement around Ch. 6. By doing the following things, you can find a lot of unique Sujimon around Yokohama;

1. Do Side-quests! There are a lot of 1-off Sujimon around town, so make sure you follow-up on all your white and blue chat icons across town. Some are in harder areas than others, so be careful and don't be afraid to run away!
2. Half-time Hero! This is a great way to both hunt for new Sujimon (Beat-up x number of specific 'Mon) and also get thrust into battle with some harder to find Sujimon (S.O.S. Missions).
3. The Underground! There are plenty of 'Mon down in the sewers, but this isn't unlocked until after chapter 5, so take your time exploring until then.

Later in the game there are more ways to unlock Sujimon, but as far as getting to 100 goes this should do you.