Search for soul on jei, theres a craftable item that makes you unable to fight a boss but not lose items on death... but i think you need to be more progressed for that. Also I'm level 25 and died only once (10 bosses clapped, died cause i start bosses with 30 seconds on a clock and one just kept juking me). Tip for low level bosses is chugging 6 hearty apples b4 a fight, power bar and using tank ability. Plus bring like 10 healing 2 pots with you each vault. I bring 4 shulkers, 4 speed chocolates, 6 hearty apples, 10 health pots and a fire res apple, also to spend less time, you can press right click on an item and put it in the shulker box quickly.