Amnesia: The Bunker

Adding this solution because I found it easier to pull off than using the grenade. On Hard mode, the beast will put up one hell of a fight in the final area, and it took me a couple hours to finally put it down. You'll need the plushie rabbit and the shotgun with at least two shells. The aforementioned can be obtained in the Roman tunnels, near the detonator handle (you'll need to defeat the crazed Beaufoy to acquire the shotgun and explore the room with the handle for the exit that leads to the bunny). On lower difficulties, you may find the shotgun in the Mission Storage room, though you'll need to run into the code for it.

Once you reach the final room, take a look at the first few adjacent pillars if they're reachable and see if there are any goodies in those. Be aware that advancing further will spawn the beast. Then, look for a wooden bridge and toss the bunny into it, preferably near the middle. Wait for the beast to get distracted by it, then shoot at the bridge. Aim for the middle. If everything goes according to plan, the beast will leap forwards, towards you, and you'll have to finish it off with a second shot.

Keep in mind that the timing is the tricky part here. Toss a grenade or shoot too early and the beast will just ignore the plushie and rush you. Sometimes even just aiming at the platform is enough to make it cancel it's animation and come towards you or fall back to a pillar. It's very annoying. Good luck!

Adding this solution because I found it easier to pull off than using the grenade. On Hard mode, the beast will put up one hell of a fight in the final area, and it took me a couple hours to finally put it down. You'll need the plushie rabbit and the shotgun with at least two shells. The aforementioned can be obtained in the Roman tunnels, near the detonator handle (you'll need to defeat the crazed Beaufoy to acquire the shotgun and explore the room with the handle for the exit that leads to the bunny). On lower difficulties, you may find the shotgun in the Mission Storage room, though you'll need to run into the code for it.

Once you reach the final room, take a look at the first few adjacent pillars if they're reachable and see if there are any goodies in those. Be aware that advancing futther will spawn the beast. Then, look for a wooden bridge and toss the bunny into it, preferably near the middle. Wait for the beast to get distracted by it, then shoot at the bridge. Aim for the middle. If everything goes according to plan, the beast will leap forwards, towards you, and you'll have to finish it off with a second shot.

Keep in mind that the timing is the tricky part here. Toss a grenade or shoot too early and the beast will just ignore the plushie and rush you. Sometimes even just aiming at the platform is enough to make it cancel it's animation and come towards you or fall back to a pillar. It's very annoying. Good luck!

you only need a grenade and a bullet.

At the End of the Game, after arriving at the Arena, locate the Wooden bridge that gets half destroyed by a Stone. Close the other bridges or destroy it after you activated the beast (Just proceed over the bridges). The bridge destroyed by a Stone has a single plank you can walk on. Just wait there for the beast. When the beast is near enough just go back the plank and the beast will jump over the gap and falls nearly of and clinging to the edge. Just shoot it. Done

you only need a grenade and a bullet.

At the End of the Game, after arriving at the Arena, locate the Wooden bridge that gets half destroyed by a Stone. Close the other bridges or destroy it after you activated the beast (Just proceed over the bridges). The bridge destroyed by a Stone has a single plank you can walk on. Just wait there for the beast. When the beast is near enough just go back the plank and the beast will jump over the gap and falls nearly of and clinging to the edge. Just shoot it. Done

Video Linked Below!
In the Final Showdown with the Beast Lure it onto one of the many Wooden Bridges as Shown in Video Below using the Rabbit Toy (Toy Collector Video is Linked as well!) Once the Beast has Acquired the Rabbit Throw a Grenade at the Beast to send it Falling into the Abyss Below.

"Beast Master"

"Toy Collector"

Video Linked Below!
In the Final Showdown with the Beast Lure it onto one of the many Wooden Bridges as Shown in Video Below using the Rabbit Toy (Toy Collector Video is Linked as well!) Once the Beast has Acquired the Rabbit Throw a Grenade at the Beast to send it Falling into the Abyss Below.

"Beast Master"

"Toy Collector"

To defeat the creature in the final area, you must have one grenade and the rabbit plushie.

Throw the rabbit plushie on one wooden bridge, and after the creature examines it, throw the grenade on him.
The wooden bridge will collapse and the creature will fall with it.