1. Start bossfights during the morning and clear the area beforehand (and destroy any spawners)
2. Make sure to be Rested, have 3 foodbuffs up and have health and stamina meads ready.
3. Always fight bosses in melee (except for Moder air phase) as it deals far more damage than the bow.
4. Adjust your weapon choice to the boss' weaknesses and resistances. (this mainly goes for Bonemass and Yagluth)