If I want to use as little current as possible, then what resistance wire should I use to get the hottest temperature?
If you currently require 28 volts and 2.4 amps, that's a power of:

$$ 28\:\mathrm V \cdot 2.4 \:\mathrm A = 67.2\:\mathrm W $$

67.2 watts. Unless you can somehow use the heat more efficiently, this is how much power you need, period. If you want less current you can do that by increasing the voltage such that the product of voltage and power equals 67.2 W. But a higher voltage battery, all else equal, will have a lower capacity, so you haven't actually increased battery life.

This is because while the battery geometry can be altered to have a higher voltage, unless you make it bigger, or use a different chemistry, the chemical energy in the battery has not changed.

The conversion of electrical energy to heat is nearly 100% efficient, so if you need to increase the battery life you'll have to look for efficiency improvements elsewhere. That means not heating things that aren't wax, such as by adding insulation or making the heating vessel as small as possible or using a more efficient battery.

There's a theoretical bound to the minimum amount of energy required to melt wax. Paraffin wax has a heat of fusion of about 200 joules per gram. A joule is one watt for one second. So if you need to melt 1000 grams of paraffin wax, you require at a minimum:

$$ 1000\:\mathrm g \cdot {200\:\mathrm J \over \mathrm g} = 200,000\:\mathrm J $$

Additionally the wax must be heated to its melting point. Paraffin wax has a specific heat capacity) in the neighborhood of 2.5 joules per gram kelvin. A change of one kelvin is the same as one degree Celsius. So say the 1000g of wax wax starts at 20°C and must be heated to the melting point of 37°C:

$$ 1000\:\mathrm g
\cdot {2.5\:\mathrm J \over \mathrm {gK}}
\cdot (37-20)\mathrm{^\circ C}
= 85,000\:\mathrm J $$

This brings the total energy required to 285,000 joules. Given the 67 watt power of your current setup, that means it must run for a minimum of:

$$ {285,000\:\mathrm J} \cdot {\mathrm s \over 67.2\:\mathrm J} = 4241\:\mathrm s = 71\:\text{minutes} $$

71 minutes for each 1000 grams of paraffin melted.

In practice, additional energy is lost to heating other things like the air, the container, and the battery, so for best efficiency all you need to worry about is heating the wax, and only the wax.