How to drive multiple LEDs from a Li-ion battery (with varying voltage)?
LEDs shouldn't be driven at constant voltage, they should be driven with specified current, leaving the forward voltage whatever it will be.

The reason is that the LED's forward voltage drops with  increase in temperature, see, for example, this white paper from Osram. As an example there, the Vf drops by 0.2V with delta T of 60C, so a 3.3V LED will have 3.1V at the same current. This means that continuing to drive the LED with 3.3V will move the working point (current) up. With a typical steep I-V curve of LED it might draw maybe 2X of initial current, which will heat up the chip more, and thermal runaway may occur.

In this particular case (24 LEDs from one Li-ion battery), one solution (engineeringly correct) is to use a boost converter, with multiple outputs into several chains of LEDs. The other simplistic "solution" would be to put resistors in series with every LED, 5-7 Ohms value, and let it go naturally.