The Cree's XP-G3's Vf will be ≈3.1V, likely less.
I will need to back off on the 0.13Ωn resistor. I now recommend a 0.47Ω
The Cree is spec's at 2.73V @ 350 mA @ 85°C
With a max 2000mA where Vf = 3.06V @ 85°C.
An 18650 at 2000ma will have a max of 4V.
Voltage will drop rather quickly to less than 4V.
Now that I know the Vf (3.06) at max current (2 Amp) of the LED and the max voltage is ≈4V.
97 resistors that will work: Digikey 0.47 3W+ Resistors
I would use the Yageo PSP400JB-0R47
This spot lens is made for a Cree XP LED. LEDil is a very good company.
Datasheet: LEDiL PRODUCT DATASHEET C11249_STRADA-S
Here is 108 others: Digikey Spot Lenses
Heat is going to be a big issue here.
I would make a custom PCB for this. These guys do a very good job and small PCBs under 4" x 4" (100mm x 100mm) are very cheap, $5 for 10 PCBS. Shipping to the US is $21 DHL. PCBway
If it is in your budget I would use 2 oz. copper, 2 sided FR4 with the immersion gold plating would be a good option for thermal management. It is much more planar (smoother) than HSAL and will help a lot with thermal conduction between the PCB and heatsink.
Qty 10 with 1 oz copper: $5
Qty 5 with Immersion gold plating over 1 oz copper $39
Qty 5 with HASL over 2 oz copper: $41
Qty 5 with Immersion gold plating over 2 oz copper $66
I would then sandwich the PCB between two plates of copper or aluminum with a hold large enough for the LED and lens. I would flood the PCB top side with copper for the XP-G3's thermal pad. I would flood the bottom side with copper and drill a lot of vias near the LED to connect the top and bottom thermal copper.
END OF UPDATE
An 18650 battery is a very good choice for powering an LED.
An LED with a 3.3 Vf works very well with an 18650 and resistor.
You did not specify the part number of lumens for your LED. My choice would a Cree XP-3G, the most efficient (185 lm/w) high power white LED. Over 600 lumens and 3.1V @ 1500 mA. Cost about $1 in small quantities. At 1500mA, 4.65W, your are driving it hard so the efficacy drops to 130 lm/w.
There are LED drivers made especially for a single Li-ion cell. For example the TI TPS63030DSKR but it has a maximum of 800 mA. Finding a driver with higher amperage will be very difficult.
The current limiting resistor is not as bad as one might think. 3.3v at 1500 mA will run hot. Hot will drop the Vf a little bit. And LED luminous intensity will drop also.
The mid-point in the discharge curve is 3.5v, so that is the value to use when calculating the resistor. That will yield an average efficiency of 94%. At the start of the discharge curve at 4V the efficiency is 83.5%. At 3.4V 97%. You cannot beat 94% with a switcher current source.
Source: hobby-hour.com LED Series Resistor Calculator
A good flashlight is very difficult to make. The flashlight I'd like to have is the Coast HP314R, a $500 rechargeable.
The non-rechargeable HP314 is only $330 powered with four D cells.
This shows these guys charge $170 for the Li-ion batteries and mini USB charger. Getting 3.25 hours of 1200 lumens out of batteries is not easy. They must be using top of the line Li-ion batteries.
Batteries last 3.25 hours @ 1200 lumens, length=17", weight 1.4 kg,b beam distance 2762 FT / 842 M.
I also like their $150 AR25R and am seriously considering buying one.
You are probably better off buying one than making one. The optics are the key.
LINK: Coast Flashlights
Home Depot has good prices on Coast flashlights.
Be sure to check out Battery University to learn the optimum way to charge a Li-ion battery. For example do not allow the battery to discharge below 2.8V. And for longer cycle life, do not charge over 4V.