How to drive high current transistor with Arduino?
6V at 100W is about 17A and you can expect a turn-on surge of perhaps 10x that. 

A Darlington, even if you could drive it, would eat 15-20% of your voltage (datasheet says it could drop as much as 4V at 25A and meet specs!). 

I suggest you use a fat MOSFET. For example, a PSMN1R1-30PL