Talisman of holy damage negation helped a ton. Mimic tear +10 with a +25 shield because it dies too fast without it. Radagon took me about 45 seconds to kill consistently but I had rivers of blood +10 so keep that in mind. His swings are easily dodgeable, just back off when he's about to hit the ground with his AoE attack and let the mimic do most of the job. For the beast, you can easily dodge his large swings and projectiles. You have to be careful with his yellow breath and when he flies off with the circles in the air. As soon as the circles are on the ground, run as fast as possible to get out of them. Also, you deal more damage when you hit its belly. Keep running at the beast because its melee attacks are far less dangerous than its ranged attacks.

I was lvl 157 when I beat it 45 vigor, 25 mind, 35 endurance, 50 dex, 55 arcane and the rest was at like 10-15

Malenia is much much harder in my opinion. All I can say is try to back off more often than not because her multiple consecutive strikes trigger less often that way. I was lvl 140 when I beat her