While I'm sure there is almost endless places to get this, the easiest and earliest place I was able to get this was during the main quest, "Jackdaw's Rest".
During this mission you will be in a cave and eventually you will come across a room full of spiders that spawn. You'll know you're at the right part of this when your character says "There's the bridge! It's almost complete!"
Spiders will start spawning shortly after you say this and this is when you'll want to avoid getting hit. After the 1st wave of spiders, 2 giant red "Thornback Matriarch" spiders will spawn in the 2nd wave. These have a lot of health so combined with hitting them, and also going for the regular spiders that spawn, you'll have more than enough time and enemy hp to get your combo up to 100, assuming you avoided getting hit.
I'd like to also add that if you don't manage to get it during the spider fight, once the spider fight is complete, the bridge will finish. Upon crossing over it, you'll come into a room full of way more enemies so you should be able to get it no problem. Even if not, there's still several other ways in game for you to get it. Another being any of the arenas you can come across that has waves of enemies for you to defeat.
I'm about 4 hours into this game at the time of writing this, but I've also been doing every side quest I see so if you only did main missions up til now, it'd probably take about an hour or so to get to this mission from the start. Good luck!