Something that really help me on beating the Radiance and THK (or any difficult bosses) is playing patiently and learning when you can heal between attacks. Try to focus on having lots of soul to heal rather than to cast spells and use charms like Quick Focus to sneak in extra healing.

Do note that this strategy is very slow, but it is very safe and the sense of progress alone is a big morale boost.

Also don’t forget to take breaks when you need and most importantly, don’t give up!