Just avoid it; it's optional.
Jokes aside, ELDEN RING's Fallingstar Beast is the optional boss at the end of the notorious Sellia Crystal Tunnel Area, and an absolute behemoth to defeat. High armour, high damage, high HP, and very difficult attacks to dodge make up the Fallingstar Beast. I hated this boss with a burning passion, and I'm well aware many others did too!
I cannot recommend fighting the Fallingstar Beast without a 100 physical damage shield, as a lot of the attacks are very tricky to dodge but should be trivialised by the shield.
Attacks you should be able to punish (and how I performed them in the video)
At 0:18 in the video, the Fallingstar Beast starts chattering its pincers; this indicates it's a grab attack, so you cannot block it. Thankfully, this attack is very easy to dodge, as rolling to the right (unlike how I rolled to the left) should allow you to get a hit or two in.
At 0:38 in the video, the boss opens its pincers and starts charging. The best way to avoid this attack is to hug the nearest wall and follow the boss while it is stampeding around the room.
At 0:57 in the video, the Fallingstar Beast's pincers start glowing with purple lightning and it begins to walk forward slowly; this indicates that the boss will raise stalagmites from the floor at your feet, so you need to run in one direction. After two, the third one is significantly bigger in contrast to the others; running outside of the range of this one is impossible, so you’ll have to time the roll with the delay. Try running closer to the boss once the third attack begins and rolling towards it, as after the attack the Beast roars, giving you room for several hits.
At 1:48 in the video, the boss quickly jumps in the air and does a slam attack with its pincers; It’s easy enough to tell when this attack is happening, so dodging it either forward or to the side should open up the possibility to hit it.
At 1:55 in the video, the Fallingstar Beast opens its pincers after swinging to the right, and then slams down; this indicates the start of a combo that leads into another slam shortly after and then a stomp with its forelegs. Block both of the slams, and then dodge out of the way when it attacks with its forelegs to get a hit in.
At 2:20 in the video, the boss lunges in the air and does a slow frontflip. When it is about to complete its frontflip, it dives straight in an attempt to deal damage. Unlike me — who failed the dodge in the video — try to time the delay of the frontflip to dodge and get a hit in. Succeeding means you can punish it, as seen at 3:27 in the video.
At 2:31 in the video, the Fallingstar Beast gets on its hind legs and purple lightning surrounds it; this indicates a similar attack as the one above, where it will raise a large stalagmite around itself. With a similar timing to the third attack depicted above, you'll need to roll as it is unavoidable otherwise. Succeeding the roll means you can get a few hits in.
Attacks you should block
At 0:22 in the video, the Fallingstar Beast jumps away and sinks its tail into the ground, dragging it and throwing a lot of rocks at the player. Although I successfully dodged it by walking right, you can't really punish this attack anyway; just raise your shield and wait for the rocks to hit you.
At 0:31 in the video, the Fallingstar Beast spins with a delayed attack. Although this is easy enough to dodge (despite me not doing it in the video), even if you do, the boss recovers really fast from the attack and continues as normal, so I wouldn't recommend punishing this.
At 0:51 in the video, the Fallingstar Beast starts doing a similar chattering to the grab attack depicted above, but instead closes its pincers and stabs them into the ground. When the boss does this, raise your shield, because it's about to launch a bunch of small rocks that are hard to dodge.
Avoid standing behind the Fallingstar Beast too much, as doing so will trigger a hind legs kick; it should be easy to dodge by walking back, but raise your shield just to be safe.
At 2:30 in the video, the boss opens its pincers and dives straight into the ground and then does an uppercut. This attack is very fast, and I recommend raising your shield, even if it breaks your posture (as it did mine) since the boss doesn't have any posture break punishes.
4:03 in the video
Throughout the second stage, the Fallingstar Beast's attacks don't change that much. That said, the attacks do become more lethal, as the boss gains the first ability it uses in the stage change as a common attack that deals a lot of damage, meaning your dodge has to be precise.
Thankfully, the attack added and the modified attacks offer more openings to punish and deal damage, which makes the fight quicker throughout this stage.
Attacks you should punish
At the stage change, the Fallingstar Beast starts glowing in a purple hue and the room starts electrifying; this indicates that the stage has changed and a grab attack is about to occur. After a very long animation, you'll have to time the attack to dodge it properly or take an inevitable chunk of damage. Whenever you do manage to dodge it, however, you can deal a couple of hits of damage to the boss, as I do at 6:25 in the video. Note that, if you manage to dodge the first attack, you'll need to dodge the exact moment where the boss stops the gravity change and rocks fall from the sky.
At 5:00 in the video, the boss does the attack depicted at 2:31; however, in this stage, the charge-up time is significantly shorter, so you'll need to get accustomed to the new speed, and you should be able to dodge and punish.
At 5:14 in the video, the Fallingstar Beast does the attack depicted above at 0:38, but in the second stage, the attack changes. After it charges around, it won't do an uppercut final attack as it does in the first stage, but will instead charge a purple lightning beam. Once you've gotten outside of the range of the attack by standing beside it, hit it until the beam stops. If you don't stand directly beside the boss, but instead your attacks take you to the front of it, try repositioning or even stop attacking altogether, as doing so will get you hit (as it happens to me in the video). At 6:14 in the video, I managed to stand beside it as I'm supposed to, and if you stand to the left of the boss, you'll get stuck at the boss' leg, which lets you do damage until the end of the attack.
After defeating the Fallingstar Beast, you'll be rewarded with a Sombertone Miner's Bell Bearing; if you give this to the Twin Maiden Husks in the Roundtable Hold, you'll then be able to purchase infinite Somber Smithing Stone (1) and (2) and Glintstone Scrap from them.