I had a Chicago Pnuematic 3/8's" unit and most lug torque (which is usually around 100 ft/lbs), it would struggle with, but would eventually break them loose. Problem was, you wore out the battery and would have to go recharge.
I quickly dumped that and bought a Crapsman 1/2" C3 cordless impact (this guy to be exact);
But quickly dumped the standard 19.2v battery for an upgraded Lithium-Ion XCP battery (this guy), which also prompted me to have to get a new charger, but...
At the end of the day, you can bust all 24 of my Suburban's lugs, torqued to 125 lb/ft (if I recall), and still not need to recharge. So it's a breeze on all 6 lugs @ 100lb/ft on the Caddy or 5@100 on the GTO, Impala or either Chrysler appliances.
Then I can use the other Craftsman C3 devices (my cordless drill, blower, etc), all with the same battery.
At 200 lb/ft of torque it is NOT the most powerful cordless out there, but will work fine for the most. DeWalt's are nice units, I have a 19v Cordless DeWalt drill too, but I think what you have is simply overmatched for what you want.