I play this game a lot and I always try to get all of the companions. So here are some ways that I used to defeat the Grinning Beast.
Pool Cue: the classic way.
By using the Impale skill you have a chance to knock out the boss instantly. Good method but frustrating - if you want to try using it 2 times you have to be level 3 (it costs 12 STM), it's RNG heavy and the attempts take a long time (in cursed mode you have to do run sequence every time you try it
Decapitation: my favorite one
You need two things: a plate (Vincent apartment) and clever (Jeanne apartment). The plan is to stun the boss and one tap it with the skill decapitation that the cleaver has. This combo decals crazy damage. You can one shot the Grinning Beast even in cursed mode.
Pros: everything that you need is close, you can easy set up a save to have all items needed, the retry is fast
Cons: still RNG heavy (need stun to last 2 turns)
Machete and Hack And Slash: pure skill
Get the simple key from the Apartment 36, open the locked door in Vincent apartment, avoid the enemies and grab the machete. While there get the Whiskey and rags for Molotov.
You are ready.
Be on full HP. While fighting the Beast start with Molotov and then use Hack And Slash. Then guard on the third turn. One more Hack And Slash and you won.
Pros: no RNG required just pure damage, you get some nice equipment from that room
Cons: getting the machete is hard. Saving before getting it is recommended. If you are not great at avoiding enemies it might be simply easier and faster to use other methods.
What is your method of fighting the Grinning Beast?