Volume, technique work! The negative is a relative prescription. We are just looking for slowest possible controlled descent. Whether that be tucked, one leg, full extended or straddle.
A: 10x3 Front Lever Negative
B: 10x3 Back Lever Negative
The negative would be without momentary pause at Front Lever, however if you have the ability to pause more power to you!
A straight body pull back up is not required, after hitting back Lever position you can tuck into a ball up/tuck position.