Correction: Roose fought multiple battles during the war of the 5 kings, commanding the infantry diversionary force against Tywin at Green fork. Though defeated, he was able to withdraw AND kill his northern rivals in the process by putting them on front lines. He later captures Harrenhal with a cunning ruse ( or roose..) and again is able to skillfully weaken his northern rivals at Duskendale. Roose is a highly experienced commander, and knows that by remaining at with his supplies at Winterfell he'll destroy Stannis via attrition. Perhaps not a good tactician, but a superb strategist.