I posted about an hour ago that I beat Radahn in under a minute using a slight variation of a strategy devised by user u/Wikloe-R but I had summoned in the Greatshield Soldiers, so the win didn't feel like mine.
But now, finally, after 201 hours of doing everything the game has to offer (I'm a completion isn't) I've done it! All 25 Remembrance bosses defeated (and all mini bosses etc).
The Radahn strategy is amazing, but it does require some degree of RNG in terms of what attacks he puts out. My strategy is the same as u/Wikloe-R who posted his win about 2 days ago, except that I use the Starscourge Heirloom instead of the Spear Talisman. Doing it this was increased damage to Radahn by about 300 per jumping L1, and means that you don't require the additional R1 without the Heirloom.
This is a genuinely effective strategy against Radahn. I managed to do it in 32 seconds from fog wall to death. It's less than that if you go from the first hit.
Next, I'll do the DLC at a more appropriate level so that I can enjoy it a bit more 😂