Let me walk you through what's been working really well against those final bosses, since this fight can be quite the challenge (and honestly, it's designed to test everything you've learned). The blasphemous blade is your best friend here, and that's because radagon has this interesting weakness to fire damage - plus, you'll get some nice healing from its weapon skill, which really helps with sustaining yourself through this marathon fight.

You'll want to focus on building up your vigor to around 50-60 (trust me, you'll need those hit points), and pump some faith to at least 40 since it powers both your weapon and those crucial incantations. Speaking of which, lord's divine fortification is absolutely essential here, because it gives you that massive holy resistance you'll need, especially for the elden beast phase.

For your talismans, think about defense and damage output - the shard of alexander works wonders with your weapon skills, and pairing that with either the holy or fire scorpion charm (depending on which phase you're struggling with more) can make a real difference. Black knife tiche or your mimic tear can be incredibly helpful for drawing aggro, which gives you those precious moments to heal or rebuff.

A little something interesting about the second phase - elden beast loves to play keep-away, so having some ranged options like pest threads or radahn's spears can help tremendously when it decides to swim across the arena (this is why having both close and ranged options is so important in this fight). Just remember to avoid holy damage weapons entirely, since both bosses laugh those off, and save your stamina for running - you'll be doing plenty of that!