Decaying dragon key brings you to 75 shields. With 300% power, mend can restore 75 kill - giving you a 1.3 second invulnerability after every single kill.
Uses 5 Forma if you don't wanna use umbral; Uses 4 forma if you use two umbrals.
The reason we want lower level [Blind Rage] and [Transient Fortitude] is because we have no need to go over 300 strength and every point in those mods increases the energy most of mend significantly.