Alright, so heres another guide/exploit by me, The first thing you want to do is engage with the boss, Now, if your losing, and feeling that your mana/stamina and health is dropping fast, No worries! All you got to do is have these required materials on your hotbar:
* Knockback Skill
* Rest Skill
Thats all you need! Just knockback the boss when your around..oh..say...65%ish, depending on how long the knockback lasts, Press the | Button (Default for Toggle combat) and then Press the Rest button, Which will make you able to rest while the boss is knocked down, This should get you hopefully back to full health, and you'll engage a weaker version of the boss now at full health!
And that, is an easy way to solo bosses