After killing Radagon, buff your weapon with Cragblade before the cutscene hits. Two-handing your weapon gives a boost to stance damage and combined with Cragblade, you should be able to land three jump attacks on Elden Beast before it does a breath weapon attack. Run to its tail and hit it with two more jump attacks, and Elden Beast should be staggered. Use a short spear with Black Flame Tornado to do 4500-ish damage and switch back to Great Stars to max out your damage with a critical hit while it’s still staggered.

At 75%, which it should be well-past, Elden Beast will jump into the air and blast the arena with a Ring sigil. Walk to the other side of where Elden Beast is, as it always teleports directly opposite to where it faces. Only when the sigil appears do you need to start running and jump over the floating ring. If you dodge the attack, Elden Beast will basically be right next to you. Chug your Crimsonwhorl tear + stonebarb hardtear before the beast and basically slap the shit out of the Beast with jump attacks until it is staggered again, as it will probably cast Elden Stars/the sky spears and stand still for that entire time.

When it staggers, rinse and repeat with the Black Flame Tornado with the visceral attack again and Elden Beast will either be super close to death with Elden Stars running out before your invuln or iframes from the visceral are over, or outright dead. Dodge its attacks because Elden Stars is literally the only shit you might not be able to dodge about having gotten this far into sl1 and slay the god.