The first and easiest place to get this, and also Combo Fanatic, is in what we can call the “prologue” of the game.
As pointed out by @RabidPiano you will have an easier time getting this achievement during the prologue if you're playing on a higher difficulty.
If you do miss this at the start of the game, there is a second point you can earn it. I'll note that down below and credit goes to @good Super man for sharing this!
The key thing to remember here is to only use your sword/physical attacks - basically only press
.
At the very start of the game, when you first meet the characters in a snowy city - you will be bombarded with monsters you have to kill.
These monsters are easy to kill and they keep coming.
You will get tips as you play that’ll tell you to use skills, but
do not do this. As using your magic abilities doesn’t count towards the combos and will reset it.
Only your physical (pressing
) attacks count.
Once you’ve got 50 and then 100 you will have unlocked both of these achievements.
If you do miss it here you can pick up both achievements by attacking the boar, which’s detailed in the other guide.
As pointed out by @good Super man There is a side quest during the second part of the game called "A Tale of the Study". There is a shade to defeat here however melee attacks don't work on it. so you can go ham and spam attacks until you unlock the achievements.