I am looking for a solution to individually dim (or if not possible switch) 30 Cree XR-E LEDs driven by a 18v 4.0Ah Makita battery and a step-down converter to go from 18v to 3,5v capable of handling about 120watts (total power when all LEDs are lit).
18v 4.0mAh Makita battery
Are you sure it's not 4000 mAh?
how can I use the WS2811 chips to control high-power LEDs?
The WS2811 will limit you to 18.5 mA. It will light up the XR-E fairly bright but nowhere near their 700-1000mA max.
If you want to use max power of these LEDs then you would need to use the outputs of the WS2811 as the PWM input signal to an LED driver circuit by using a resistor rather than an LED connected to the outputs.
The Texas Instruments TLC5973 3-Channel, 12-Bit, PWM Constant-Current LED Driver with Single-Wire Interface would be a much better choice than the WS2811.